Astarion's clearly experiencing some regret / guilt post ascension, and from his behavior we can see that he's projecting his anger at himself onto the PC. He's not trying to win the PC over in that moment. His goal is to provoke them into showing their "true colors" through a love-test designed to be impossible. He wants the PC to lash out at him and call him a monster. If they do, he can write them off entirely and absolve himself as the victim in this situation for having been manipulated by a PC who supposedly "loved him".
So, that's kinda the issue. There isn't a way to smooth things over. His goal is to hurt you, plain and simple. No amount of agreeing with his decision or reassuring him should necessarily be able to change that. The only way to prove it to him is to become his spawn, an act that is essentially surrendering your boundaries and control as a show of loyalty.
Given all that, what is missing? A character who is happy with the outcome and wants to maintain the status quo should probably be calling out his negative attitude / demeanor, but if you want to ignore it, fine. Either way, it should still run up into the same ultimatum: become my spawn or we're done. I could see some attempts to reassure him that what he did shouldn't weigh on him or promises that you're loyal no matter what, but those are all gonna run up against the "prove it by becoming my spawn" test still.
I think what would really help is a:
So, tell me what you desire. What can I do for my favorite pet?
> 5. "What is this really about Astarion?"
that can quickly get us to some version of "You made me this, if you really love me, prove it! Become my spawn!"
I don't remember any instance where he would show regret, he won't stop talking about how good he feels and how he wants Tav to share in all the splendour. The only provoker in this situation is Tav. You could roleplay your chara being fully on board with his schemes throughout the game but in this single scene suddenly you're not anymore. There is zero conflict if you tell him you want his bite or his body. If he is provoked that's when he actually lashes out at Tav for being a hypocrite (and rightfully so).
Tav is the last piece of the puzzle to his vampiric dream. If he convinces Tav then in his eyes he becomes better than Cazador, because he never had anyone who didn't hate him, actually cared for him and someone he could truly trust. I don't think he ever fully trusts Tav until they become his spawn. Even Cazador will say that to Tav if they go to him alone. That Astarion's distrust in everyone is his only redeeming quality. Astarion pretty much admits it by telling Tav they'd earned a little bit of trust, but only a little. In a banter with Lae'zel post-ascension he tells her he needed someone he could trust and that now he knows Tav will never turn on him. There is another one with Karlach where he says he has one person who trusts him completely and that's enough for him. Spawn Astarion tells Tav he'd be ok with them going their own way, which is a lie. He's terrified of being alone after that year of silence. I'm sure if he could he would prevent them from leaving him like AA does.
To me it's the other way around, he approaches Tav because he's confident enough they would agree to his proposal but also worried they would change their mind once the dust settles and they actually do see him as the monster he thinks he is and break up with. If he manages to secure them before they possibly start feeling guilty then he has nothing to fear anymore.
If Tav does break up with him, he never blames them for helping him become the vampire lord. He bashes them for rejecting his gift and not wanting to enjoy life with him, and tells them they'd regret leaving him.
I would also like to see the option to call him out and ask what it's really about. I think he'd say what he does post-breakup that he was really trying with Tav in the only way he knows how. The ultimatum should also stay there, however, because there exists an ending where he lets Tav stay mortal for a while, there should be an option to ask him for some time to decide. I think he would be impatient and just give them one day, approaching them the next long rest.
That being said, you do have options to tell him you did it because you wanted what was best for him. It's literally the first dialogue option available. You can also tell him you wanted him to be free, but it seems now he never will be. The first part reflects the player's intention and the second reflects the narrative outcome. I understand you can play a character who doesn't believe that, but they're trying to balance between the freedom to roleplay and story integrity.
The way the dialogue is framed makes it seem like Tav is preparing to break up with him and is starting off gently before getting to the meaty part. If you could roleplay agreeing with him all the time, you should still be able to do that here. This is plain simple imposition of the writer's idea of Tav onto the player and taking away roleplay from a roleplaying game.
If we go down the angsty Tav route, why can't Tav actually feel sorry for him and say "What have I done? It's my fault you're like this now, I've created a monster". Heck, you could even roleplay Tav as a self-sacrificing hero who wanted to help him but then threw the morals out the window for him, and decides to spend eternity with him as a form of punishment for their crime, while hoping they can still guide him to not go full evil.
There isn't a more genuine romantic moment because they're intentionally portraying the start of an abusive relationship. And it seems like people are really looking for a version of the relationship that isn't abusive, but his being trapped in the cycle of abuse is the point. It's part of the conclusion to his story.
That's a very finite and black-and-white thing to say. He is as trapped as he allows himself to be. At that point the only thing in his way would be a mental blockage. There are no external limiting factors anymore. I think the story is more potent and interesting if he becomes powerful but proves he's not another Cazador, instead of being scared of his own shadow and denying himself just to please Tav.
