After finishing the game with romanced Astarion I have to say this is my favourite character and romance in any game I've played, so thank you Larian for giving us such a delicious treat!
Despite the romance being generally fulfilling there are still things that could use improvements and some extra dialogues and scenes are sorely missing. Here are my observations and suggestions after finishing my Tav's story and watching vids of alternative routes, organised according to the acts of the game.

Act 1
1. The mirror scene - after it or during the scene as an alternative approach Tav should be able to suggest to Astarion to use the tadpole so he can physically see himself through their eyes, just as he sees
his scars during the ascension scene.
Another possibility is teaching him to conjure up a mirror image if Tav is magically inclined or asking Gale to help out. Let Astarion finally see his own face!

2. At the end of the first romance scene triggered before the grove celebration Astarion asks Tav to leave before more people thank him for saving their tails, which is clearly a thing from early access where it was impossible to be with him before the grove battle. He should have a different line there.

3. When propositioning Tav the second time during the grove celebration Astarion says he'd find them after everyone was asleep. When they eventually meet up they are actually standing right where they saw each other last (in my game I still saw people walking around in the background). I think they should reunite either while exiting the camp or somewhere outside of it altogether. What would really elevate this scene is some extra playfulness, for example Astarion grabbing Tav's hand and sneakily running off together (they should be more comfortable with each other at this point after all) or, if they do meet up in a different location, Astarion could pull Tav in for a kiss.

4. My Tav was a drow and I honestly cannot remember him ever making a comment about it. I was expecting him to be a bit mistrustful due to drow race's backstabbing reputation and asking her at some point if she intended to treat him like a glorified pet as is customary in the majority of their matriarchal society. There should have been extra RP options for Lolth-sworn characters to be more intimidating and dominant and maybe him showing some discomfort about it as a prelude to his trauma reveal and for Seldarines to say they reject the rules of that society and see him as an individual and not a plaything.

Act 2
1. During early access I remember seeing a video from mmogurl where Gale propositioned Tav knowing they had already been involved with Astarion and it was possible to refuse him on the account of not wanting to hurt Astarion and Gale even commented that Astarion was lucky and that loyalty like that was a rare thing. In the release version I saw no such dialogue, in fact Gale invited my Tav to see the starry sky, professed his love and there was zero mention of our lovely vampire. The only way to refuse Gale was saying we didn't love him back. It would be nice to have that option restored. For science, I tested what would happen if we kissed Gale but not gone all the way. He then talks about being excited that Tav is in love with him (Tav never said it) and how wonderful their honks was, which never even happened lol. Afterwards, Astarion wants Tav to choose between them, and when he is picked, he is happy telling her she can consider herself well and truly taken. It looks like Wyll is not informed about that because right afterwards he tries to seduce her with the dance scene. After kissing Wyll there is yet another dialogue with Astarion asking to choose him or Wyll, finished on the same note about Tav being properly taken. It looks like the romance flags here are completely wonky. And if it's actually supposed to be this way somehow, Astarion should be upset with Tav for getting involved with someone else when they're supposed to be a thing already. Interestingly enough, Gale only propositions Tav before talking to Araj Oblodra, afterwards there is no projection of his during the evening and Astarion can safely confess to Tav. The dance scene with Wyll never triggers then as well.

2. When killing the
rat figure before encountering the orthon Tav has no way to honour the deal with Raphael once they start talking to Yurgir. Astarion gets very angry and has the same reaction as he does when agreeing to help the orthon fulfill his contract before finding the rat person. He should definitely have different lines because at that point Tav has literally no clue they are sabotaging Astarion. Perhaps Astarion could realise his deal is about to become void when Yurgir explains what is happening to him and urges Tav to kill him quickly before Raphael shows up to collect the devil.

3. The confession scene after talking to Araj Oblodra should be expanded and reworked. The only way for him to express his feelings is to ask him why he slept with Tav. It would be nice if he said how he felt no matter what. In this scene he also never tells Tav that intimacy with them still feels tainted to him, which is a crucial part of his romance so the option to tell him to abstain from sleeping together seems to come out of nowhere. We can easily headcanon here they never take a break, which is not what actually happens.

4. Apparently, after the 'I want us to be something real' confession scene we can still force Astarion to bite Araj and he won't have anything to say about it afterwards. That's an oversight.

5. Astarion needs a heartfelt, infinite hug option after his confession or at least during more painful parts of his storyline.

6. I'd like to hear other companions make comments about Tav and Astarion's relationship after they decide to be something real. Some extra camp gossip is always fun!

Act 3
1. It would be nice to have a small conversation with Astarion after the love test with the dryad to hear what he has to say about her verdict.

2. When Halsin propositions Tav and we ask Astarion what he thinks about the idea, there needs to be an option to tell him we only wanted to hear his opinion about it but had no intention of going through with it because he's the only one for us and then be able to reject Halsin despite Astarion's theatrical approval. He does say he'd wondered when Tav would mention it and had seen Halsin's yearning since the grove itself, he deserves some extra reassurance of love here after all the anguish and uncertainty he'd quietly gone through.

3. If engaging in polyamory with Halsin, Astarion has zero dialogue afterwards, which is rather strange. It's only logical they'd both want to check up on each other how they're doing. Astarion also wouldn't fail to make fun of Halsin some more and ask Tav about his performance. Tav could say that
a) it was everything they wished for
b) it was wilder than they expected and they may need some time to recover from it (the bear form exclusive option)
c) it was good and they're curious to see what happens next
d) it was fun but nowhere near as fun as with Astarion, but for now they'd see how it goes
e) it was fine, but they kept thinking about Astarion throughout the night and are still unsure if they want to keep it as a one time thing or keep going
f) they regret it, it doesn't compare to the bond with Astarion and they would break up with Halsin the next time they talk
g) they didn't do it after all (if Tav changes their mind during the scene)

4. Astarion should have a conversation post- drow twins scene - he is clearly not ok during the act and Tav should be able to comfort him afterwards.

5. The Haarlep scene - Astarion should honestly be shouting at Tav and asking them what the hell they are doing.
He was abused for centuries and seeing Tav getting coerced and selling themselves like that should make his blood boil and evoke a very emotional response. There needs to be a persausion check to tell Astarion to calm down. Even then, if he gets persuaded to not interfere, after Haarlep gives away certain secrets and prepares to leave, gloating about misusing Tav's body, he should sneak up on the incubus and murder him in cold blood before he gets away. Spawn Astarion should do it very angrily and ascended with cold precision saying something very mean to him while shanking him.

6. After meeting
Araj again in act 3 and being asked to drink the disgusting potion, Astarion tells Tav to dispose of the bottle but I think he should also protest and pressure Tav into refusing to take the bottle in the first place. The potion would mess with their blood and since Tav is his little treat he would not want their 'bouquet' to be contaminated with filth. In my game I accepted the bottle, destroyed it and told her about it. Astarion swiftly killed Araj when she turned hostile but to my disappointment he had no comment about what happened. As an ascendant he says he wants to see her dead before entering her house and as spawn that he pities her. He should either be gloating over her corpse, or feel somewhat remorseful but also relieved depending on his identity.

7. Considering
Raphael's involvement in Astarion's storyline, him being a devil and Mephistopheles's son, there really needs to be a conversation where Raphael is in awe if we went through with ascension. Per his own words - the ritual was diabolical and so vile, no one'd ever done it before. It should be a big deal to him that it actually took place and he should also have some extra comments towards vampire lord Astarion if he's in the party. I imagine he might make some snide remarks about spawn Tav as well, after all he trashes them for romancing the Emperor. Maybe even give Tav a sick burn in response when stealing the hammer from him?

8. After ascending Astarion
and meeting the Gur, when Ulma asks us to side with her and kill Astarion, we should have a romance specific line saying something like 'I will not let you lay a finger on my lover; back away or die.'

9. Post ascension conversation when Astarion asks Tav how he can reward them should be a bit longer with Astarion using more persuasion techniques. Secondly, this scene could use some extra options.
In act 1 he mentions feeling Tav's shakes of excitement, right after ascension he can hear their heartbeat and them holding their breath - clearly he can read Tav's body language easily (plus the worm connects them) and he should say something similar here too, like he can tell they want it but they're too scared to follow their desires, sneer that they still choose to be naive (reference to early act 2 dialogue) when they can become more powerful and never have to worry about losing him and dying of old age. What's more, why can't we tell him we would like to go on a date with him? Spend some quality time together away from the rest of the group? We can only be thirsty for his body or his power, lecture him or be upset with him. Some more romantic and playful options would have been neat too. Of course, he would still spin the conversation so he can offer vampirism. An option to tease him if that is his idea of a marriage proposal would be funny. A serious one to be able to call him out on him manipulating Tav and pushing them to make a decision and demanding his sincerity is also welcome, where he'd reply similarly as he does post rejection where he mentions he was really trying with Tav and doing things the only way he knew how.

10. It's possible to finish the game
without turning into a spawn and still staying with ascended Astarion as a romance partner by jumping through hoops, but for the sake of extra options it would also be nice if we could tell him we wanted some time to decide when he offers to turn Tav the evening after ascension. He could at least give them one day to make up their mind, that's not too much to ask for. It would be nice as well to be able to discuss it with other companions to hear what they think about Astarion's proposal.

11. When Tav joins Astarion in immortality their look doesn't change whatsoever. We can use the magic mirror as a workaround to change Tav's eyes to red and make their skin lighter but they will still miss vampiric fangs.

12. Everyone should have something to say to Tav about them becoming a spawn, it's a drastic and permanent change for the character, same as DU choice. There is this one dialogue with Karlach about Astarion if we talk to her after the ritual. However, if we trigger it before becoming a spawn we miss out on her input about Tav.

13. It would have been cool if there was a new option for mortal Tav who rejected ascended Astarion's offer to have a change of heart before the end of the game, for example after dealing with the Chosen. It could be a scene during a long rest where Tav wakes up at night reminiscing and regretting how things turned out (Astarion does tell Tav they'd regret leaving him, let's see that actually happen!). The narrator could then ask distressed Tav if they wanted to approach Astarion about it or not. After selecting 'yes' a dialogue option would open up where Tav would be able to tell Astarion how they missed him and realised their mistake, how they didn't want to have a future without him, etc. I reckon he'd need some persuasion checks to forgive Tav but eventually he'd cave in. It would lead to the same romance scene but he would be a bit more pushy and even aggressive if Tav hesitated. For some extra angst, perhaps, similar to the outcome where he refuses to break up with Tav, and only if he was provoked sufficiently enough, he would actually attempt to turn Tav no matter what (he admits in a threatening manner he should have done it after the break up - I imagine in this scenario he would lose his restraint). Tav would then decide to give in or fight him. There could be a constitution or strength check where Astarion succeeds or fails and in case of failure he'd leave the party permanently. After succeeding, the morning afterwards there would be several options to initiate a dialogue, including a throwback to the time he kills Tav while drinking from them the first time with angry Tav wanting to punch him and complaining he killed them yet again.

14. It is not entirely clear who Tav is as a vampire. Astarion says they're his spawn, yet Tav can actually use the bite on him, which would turn them into a true vampire, according to Astarion's own testimony. Unless that only works when he consents to it with an actual intention of making Tav a true vampire? Moreover, I heard of theories that suggest that he actually turns Tav into a vampire bride by offering his blood during the transition. Although it is only a drop of blood, but maybe it's enough for the ascendant spawn? If Tav is a bride, he actually shouldn't be able to compel them, because even without the tadpole they would keep their free will, but of course he could still manipulate them into thinking he can and use physical force and magic to keep them nearby. Or maybe as ascendant he can still compel even a bride and a true vampire? Too many questions.

15. Spawn Tav never has the option to tell ascended Astarion they love him if they stay with him till the end. He still deserves love and kindness, right?

16. The last kiss that should count for something is ridiculous. It's the same chaste peck on the lips like all the other kisses Tav can share with him! It should be unique and memorable.

17. There is a scene where we can have a discussion about becoming
an illithid after accepting the worm from the Emperor.
However, Astarion has nothing to say to Tav after
freeing Orpheus and taking a moment to decide if they will sacrifice themselves. It's unbelievable if we blatantly refuse the Emperor right away but also quite bad if we did indeed have the previous debate. It is his last chance to convince them to remain his beautiful lover and he's silent? No way! He's never particularly fond of Tav playing a hero and he should be desperate to keep them away from tentacles at that point; with ascended being all 'Don't you even think about it! You are NOT doing this and turning into a slimy freak. I've given you immortality, just think of our glorious future together! Surely you wouldn't discard my love and my gift just like that? I don't care how many people have to die there or if I personally have to shove a worm inside someone's skull, you will NOT throw away everything we have and CAN have for some dumb heroics. I will NOT lose you to this.'; and spawn 'You MUST reconsider this. Think about what will happen to you, to us. We've come so far together, I don't want to lose you, I don't want you to lose yourself! You will no longer be YOU. Leave heroics out to someone else this time, alright? There has to be some desperate hero out there without a beautiful vampire lover by their side who can have this dubious honour in your stead'.

18. Romanced ascended Astarion as one of the allies in the final battle should have a unique response to Tav when offering his support, for example: 'My dark forces are at your disposal in the battles ahead, my love. Our foes don't stand a chance against our might.'

19. Ascended Astarion tries to persuade Tav to
control the brain so they can rule together
but it's actually not possible in the game and he will become
Tavzador's thrall for eternity. If the Chosen could rule together, why can't Tav/DU and him?

20. The 'good' spawn ending is depressing and disrespectful to Astarion.
He sacrificed himself for the dubious greater good, acting completely against his own nature and he's rewarded with lame comments from other companions and indifference from his lover, while he runs for his life and, as depicted in his origin ending, ends up completely alone, sulking behind some cargo in a fetal position. #JusticeforAstarion!

21. The ending conversation
with ascended Astarion is well written and his excited laugh at the end is a chef's kiss. One thing however that I found questionable is Astarion saying their enemies had fallen but then Tav has the option to say 'let's put our enemies to the sword'. Which enemies? Aren't they already dead? Meanwhile, spawn Astarion's interaction is just... sad. He is still very insecure about their relationship despite him and Tav already having decided they want to be committed to each other during the graveyard scene - the dialogue should reflect their previous declaration. After abandoning the ritual it's understandable he seems lost and unsure what to do next, but at the end of the game he should at least have some kind of idea of what he'd like their future to look like, yet he still has no plans and even suggestions and basically leaves everything up to Tav to decide if they stay together and what their next steps are. Despite him saying they are equals, it really doesn't look that way.

22. The 'good' ending conversation should generally be more intimate and engaging. It doesn't even look like they're truly celebrating their victory. They stand away from each other like strangers and are quite business like. Why won't ascendant hold Tav in his lap and spawn cuddle up with them on the bed with his arm around them, why don't they hold hands for a bit while they talk, touch each other's arms and faces? Actually act like people in love who are relieved they finally have a moment to themselves after all the hard work?

23. Spawn Tav/DU
who broke up with Astarion before the end of the game and is alive, not a squid, not involved with anyone else and not gone to other planes,I think should have an ending scene with him where he decides to reeducate them and remind them they made an eternal commitment and are still bound to him. He should offer them one last chance to get back with him, and if initially refused he should threaten them with confinement until they change their mind or simply say they'd have to stay with him regardless as his thrall and right hand and they can only choose to make their life more or less pleasant.

24. Mortal Tav/DU
who refused to be his spawn but accepted to be his partner in crime should also have a unique ending with him where they scheme together and Astarion would cheekily ask if they are still sure they don't want to mix business with pleasure and Tav can admit willingness to get back with him and even becoming his spawn immediately or eventually. This scene should also occur for illithid Tav, but play out similarly to the romanced spawn Tav who became an illithid with Astarion refusing to have any romantic connection with them.

25. Some epilogue cutscenes or slides would be great. For example,
ascendant redecorating the Szarr mansion with his consort (he hates the distasteful art and ugly carpets after all), throwing a party, sitting on a throne together, spawn burning the mansion down, Astarion and Tav leaving Baldur's Gate to travel the world, buried in books investigating how to make Astarion walk in the sun, settling in somewhere, etc.

26. When evil DU
prepares for the confrontation with Bhaal in the 'good ending' and Astarion can indeed compel them, he should notice something is off about them and use his powers in that moment to fight with Bhaal for control over DU's murderous instincts. DU should have a saving throw where they either submit to Bhaal or Astarion or stab themselves.