Well, partly that there's no way Astarion wasn't at least a little bit of a prick then. You don't turn into the type of personality he is in the game without at least a foundation in narcissism and spite. The way the game plays out his lore, pre-spawn Astarion was at least similar to the typical patriar of the city in behavior and beliefs, and that points to the usual fuckery a ruling class is prone to.

That would keep it from being unbearable. It would let the story show him developing internally the compassion he eventually shows once his barriers are down (assuming you take that path in game).

If I were writing it, I'd focus on him initially looking at it as a good thing, another layer of power and influence, then being stripped of the pretence of any self determination at all, but building back up that kernel of defiance that kept him strong until the events of the game.

There's hints of that kind of internal process in the game, and a ton of room to still play around in it so that it isn't just essentially fleshing out established lore with fluff.

But it really depends on what the goal is from the owners of the IP, and how much they want to micromanage it.