I think the wild card for the roleplayer is the fact that Astarion is undead. So how do undead express their emotions and handle trauma ? I guess there's a wide landscape of possibilies to choose from.
Take the "romancing" example. Astarion certainly likes sex. At the grove party he tells you so much. Can he also "love" ? Probably not in the same way as a mortal. But Dracula had 3 brides that he cared for well. (Untill he abandoned them to rush to his new fascination, Mina) He cerainly didn't seem to treat them badly, as Cazador trated his conquests. So Astarion could get infatuated with a party member. Have sex with them, care for them and try to get into a stable and permanent relationship. Like Dracula, I think he would want to turn the companion to a vampire also. But, if the love were reciprocal then..... why not ?
Just a quick note: that Dracula thing was made up by Coppola, it's not in the original book.
As for the topic: you can use some headcanon: Astarion having to take control because the group somehow thinks, he is better suited than them and letting him make decisions is a bit similar to Durge tbh. For me, it's that him taking responsibility for the first time in a long while and maybe getting positive feedback can change him to become a better person. He clearly cares about Sebastian and he is not immune to the fate of the children by act 3 so him taking control and people actually applauding him for that, can change.
Plus playing as an origin is making that character your player character, there is no 'This has to happen' , just see it as a clean slate and make that character yours.
