I'd like to add: If Astarion Dark past is going to be realized and a dialog will be added too. I realize that it may never happen at all and so it's a very dreamy suggestion. But I'm too inspired so bring it on. The playstyle should significantly affect this dialog. It's an important thing. Speaking of playstyle I will use the terms alignment.
Astarion, before the ritual. And in general. Astarion has no reason to care about his dark past if the player is dark too. This system works in the conversation about controlling the cult in front of the Moon Towers. That the playstyle is evil\neutral-evil, or really neutral - doesn't interfere. If the player tells Nere to "kill the slaves" or "no action", for example, there are several key moments throughout the story that will signal Astarion how to act and shape his attitude towards the dialog and the player. There's a subtle point with neutrality, neutral alignment doesn't take sides, chaotic-neutral doesn't either (but is more subject to own caprice). And for Astarion, that's going to be I think the hardest part, understanding a player like that, as opposed to hero and villain. I think he'll be helped just here by the fact that the player is in a romance with him. As in the case of the story there will be key-romantic-moments. And not just good\care ones, but sacrificial and strange ones. One of the first is the player letting Astarion drink his blood\ or let kill player and not being too mad at him in the morning. That is moments when the player did something just for him risky things, questionable, supported not-good words Astarion. That is, Astarion would understand that he's the player's weak spot, he's being forgiven everything, so his behavior would be different. Or the player is quite individual and Astarion's influence is obviously less. Also the impact of "breaking up" when the relationship was before.
A player should be able to part ways, in every possible path because of this.
After the ritual.
Spawn. This will be an opportunity to show the thorns and dark side of Astarion Spawn. If for example the player has done some pretty evil things and doesn't care about the past, or evily-evil says Astarion was doing an interesting venture. Someone wants keeps playing Boney and Clyde without the ritual. Also a good player, could still condemn it even before going to a painful breakup. Or take great indignation if an evil player judges it.
Lord Astarion. DnD5 true-vampire rules work on an ascended vampire? In the interview said it's still Astarion. So I think, we can only evaluate actions. For Lord, it's a chance to show his gentle\fun side. For example, player has neutral and is simply Lord Astarion's own moral compass. But not in the sense of breaking up. Just a little... directing and structuring that chaotic fire, in Lawfull Evil so to speak, when there's a romantic relationship it's all the more interesting to watch. I like the tropes of Unholy Matrimony and Even Evil Has Loved Ones, which there just aren't many of these days. If the player was a good\neutral and said something nice\neutral or even supportive about Astarion past. That is, the player has stepped to the dark side. Lord Astarion will have a lot of branches - spawn Tav, after the ritual right away, after the breakup, after agreeing to be "partners-in-crime", even mortal Tav at the end maybe.
In general, I want a strong correlation with actions, how the player behaved in the game in any path for Astarion: in key moments, what decisions were made, how they behaved with Astarion, including in the romantic sense - it will be a deep dialog. With different options, including based on the player's race, class. Mindreading, Astarion's facial expressions and narrator's voice - that would be interesting, too.
