Oh yes, I see another fellow Astarion enjoyer. So here are my 2 cents and very heavy spoilers, so be warned:
1. It has been said over and over again, act 1 is a masterpiece, nothing bad to say about it other than Wyll's story rewrite is quite... odd. He still approves of slaughtering gobbos yet all of his backstory with them is completely gone? Why? It expanded his character so much, other than being 1 dimensional goody-two-shoe type of character. I loved the EA scene with Spike and with torturing that one guy (or killing all gobbos in the room before they touch the drums).
Act 2 is really damn good but it's a bit too reliant on dragging SH everywhere. She literally has reactions to everything. Also, some quests love to break if not done in a proper order. That should be rectified (like we had so many approaches to finish certain quests in act 1 and all of that is nearly gone in act 2 and especially in act 3). Ketheric is a goddamn highlight, amazing VA work and an amazing boss fight,if... kinda dumb, I mean, a bunch of lvl 8-10s fighting an avatar of Myrkul? Really? That thing should wipe the floor with us the same way Bhaal snuffed Durge out and/or the same way Vlaakith can just wish us dead at the creche.
But to sum act 2 up I would call it: SH and Ketheric show!
Act 3 is where a lot of the story just falls flat on it's face. First of all - the Emperor and the black hole of plot holes every scene he's in creates. I am totally okay with the 'benevolent' Guardian turning out to have been a mind flayer all along (even though I preferred EA's daisy), what I am not okay with is this:
1. He's supposed to Balduran, right? So why in the cutscene he shows us of his time before ceremorphosis we can clearly see the titular city in the background? Fully constructed? I mean, Baldur's Gate is what, 500-600 yr old as of the timeline BG3 is set in? Balduran was an adventuring sailor who happened to find his way to uncharted lands across the sea and came back to the Sword Coast with untold riches, built the Baldur's Gate (as in the actual Gate leading into what would later become the city). The city didn't just get built in a year, or 2, it took decades. So how old was he when he returned to the Sword Coast from his 2nd voyage he allegedly disappeared in? Yes I know, he was an elf so he could live up to 750 years. He talks about the MF colony that snatched him being under the Moonrise Towers, so he did return to the SC/BG in the last 100 years cause Moonrise is what 150 years old? Tops? Let's say he was a mind flayer for ~100 years. How come he didn't announce himself to the city upon his return originally, before exploring Moonrise? He does strike me as a bit of an attention seeker. Oh and also, wouldn't that mean that he was a mind flayer around the time of BG1? That he might've lived through the aftermath of the Time of Troubles? Also, wouldn't that mean that Ansur was in/under BG for a long ass time before dying and nobody even noticed? Hmmm. Sure, dragons do like their humanoid forms (Firkraag, Adalon, Abazigal) I suppose. Still no reason to not reveal yourself as the city's great protector at least to some people.
2. Stelmane. So when you are a meanie to the Emperor when he just shows up half naked in your dream (naughty naughty) and distrust him consistently, he shares with you a memory of how he caused Stelmane's aneurysm while also calling you a puppet. Okay. Being called and used as a puppet I can take in stride. Fine. How come is it never ever brought up again? You can't discuss that new information with anyone, not even with Emperor himself. WHY? I mean, there were enough breadcrumbs: like finding Stelmane's doctor's journal that also leads you to conclude that the 'tall robed figure' did something to her. Just like Ansur revealing him to be the long lost hero Balduran can never be discussed with the Emperor ever again, but at least Wyll, Wyll's father and the rest of the party do have some thoughts to share about this revelation. Just not about Stelmane, eh?
3. Endgame choice. Enough has been said about his entire character falling apart the moment we choose Orpheus, it's appalling and completely out of damn character. And most frustrating of all? He can't be stopped. You can't toss a damn spear in the portal he goes through in hopes that maybe he gets hit? Not a chance to talk him out of it? "You choose Orpheus? KKTHXBYE" like what is this writing?
4. Companion quests pretty much leading nowhere, aside from Wyll's (cause of ties to Ansur and the Emperor) and Lae'zel's (McGuffin chase of Orphic Hammer) that have direct ties to the main story and improve it, somewhat.
Karlach wants to cave Gortash's face in and it's totally deserved (and understandable). SH's ties to the main story kind of disappeared the moment act 3 rolled around (which is so damn Gith-centric it's crazy)...
And here comes the biggest offender: Astarion's plot, while I absolutely love his character, Larian did the entire vampire plot of Cazador such a damn disservice. Cazador comes off as a damn
petty clown during the encounter, not this all powerful,
smart force of political power and intrigue behind much of the Upper City. I guess that part was cut cause, yeah, no Upper City access for us in the game as it is. Such a wasted opportunity. What I expected is a welcome to a ball in session, to some display as to why Astarion fears Cazador so much... You know of this writing technique - 'show, don't tell'? We've been told by Astarion the entire game how awful Cazador is, but we've never seen it, meanwhile this technique is constantly utilized when it comes to Dark Urge's story events, which were absolutely disgustingly graphic (and props to Larian for not holding that back!). Anyway, back to Cazador, imagine how amazing it would be if we'd be forced to play along to his little farce of a ball (otherwise a bunch of 'innocent' nobles in audience would die along with Cazador instantly snatching Astarion away for his ritual) like think of all the possibilities! But no, Cazador's a laughable clown of a villain. Ugh.
5. Dark Urge's reactivity in act 3... don't get me started. It's all more or less fine until the duel with Orin, then the game reaaaaaaaaaaaaaaaaaaally drops the ball. Companions barely react to anything. Not EVEN YOUR DAMN LOVE INTEREST. When Bhaal snuffs Durge out, companions, especially LI's should have a reaction other than standing around with a shocked expression on their faces and weapons drawn. LI should rush to Durge's body, crying, screaming, trying to resurrect them and other companions could act similarly to the way Joker in ME3 gets told that Shepard ain't coming back and that they have to go, only for Withers to swoop in with a miraculous solution.
I have posted these pictures in another thread but I will repost them here, under spoiler tags cause they're taking too much space as it is with an explanation why. So Astarion has totally different reactions to certain events in act 3 totally depending on him being ascended or a spawn, an example of the conversation after Ailyn defeats Lorroakan:
He also has similar reactions (that are different depending on his vamp lord vs spawn status) to meeting Araj Oblodra again in act 3, that short Haarlep cutscene if you slept with him in HoH, conversation after slaughtering Raphael and Ansur, being talked to in the end game (Morphic Pool/Astral Prism/Upper City ruins). But when it comes to Durge's pivotal story moment, both of his versions have the exact same thing to say?
And while with Haarlep using Tav/Durge's body - Astarion had 2 separate line deliveries even if he said mostly the same thing (and the dialogue wasn't an exact copypaste - it's clear to tell which is spawn and which is ascended) - there are examples of that on youtube as well. Well, this pivotal Durge moment has none of that. Even better, I decided to check what happens if you accept Bhaal (aside from Jaheira/Minsc leaving) and spawn, allegedly good Astarion still had that line about "Bhaal's dowry"! Like my dude! You should be disgusted, not happy! I am pretty sure all romancable companions have the same issues in that scene, I would be not surprised if Sharran vs Selunite SH had the exact same responses to this event as well!
But really, when it comes to Dark Urge's story I really feel like it was disentangled from EA's Tav (Tav had the urges to chomp on Daisy and also that dream of BG on fire screams very much Bhaalspawn to me), made extra bloody (rather than being just some rando Bhaalspawn on a quest of self discovery perhaps), added connections to the city via being the mastermind together with Gortash behind the Absolute scam. Maybe someone in Larian thought that twist was too similar to Gorion's ward's story? Altho, imagine how spicy it could be if basic Tav was a Bhaalspawn and didn't even know it... Currently in the game as it is, basic Tav meets only 2 out of 3 avatars of the dead 3: Myrkul's we fight, Bane can be spoken to via speak with dead spell on Gortash (and funnily it has no voice over

) and Bhaal is only available to Durge. Quite... odd.
6. Party banter from that vid of Dana Duchy you linked, yeah, I did hear a few of these banters super early in the game, mind you those were companions flirting with each other: I heared Wyll complimenting Lae's eyes, then he complimented SH and she said she was not interested cause she heared what he said to Lae, Astarion calling SH a flower, then Astarion enjoying the walks with Gale and oh, there were a few that Dana did not post, that had alterations depending on whether Astarion tried to bite Tav and revealed himself as a vamp spawn or not. Yes there are banters in the game that get altered by that! But I have heard none of companions commenting on Tav/Astarion's growing romance. Nothing, the entire game! I believe it has been bugged, or a switch wasn't flipped by Larian to enable these banters. And such a shame too.
Speaking of Tav getting KO'd/dying in combat, I barely hear companions commenting on that. On both runs. I sometimes hear Astarion shouting "Oh no my sweet bloodthirsty friend!" and I am like, don't you have anything else to say about Durge you are in romance with other than "friend"? It was even worse for basic Tav who romanced him.
7. 'Epilogue' choices... so, why romanced spawn-Starion still needs to be reassured that I do in fact still wanna be with him? Wasn't that already established a bunch of times already?
1. At the graveyard scene
2. After Durge gets resurrected (sorry basic Tavs)
3. Morphic Pool/Astral Prism/Upper City ruins mini conversations
Not to mention the travesty of him running away burning and that being treated as some sort of comedic relief? Nay I say.
Also the entire debacle of who gets to speak in cutscenes... here I have an idea: rerecord a bunch of lines and make them all have back and forth conversations after defeating the brain? So everyone gets to say something? And prior to that, make it so everyone in the party always says their piece in cutscenes. Here, done! Problem solved.