Hello! As we already have vampires in game, you may choose to go with how they play out the whole 'sunlight' issue: burning is still instant and noticeable, but not lethal. However, there is very little to no sunlight at this time of year — which is something in Astarion's favour for now, at least. And that in future, he may be fine in very overcast days, with very little direct sunlight, meaning he can still be out during the daylight on some days!

Blood thirst can still 100% remain with him in game. The Interview With The Vampire boys are currently feeding from animals or otherwise half-starving, so that's definitely something you can have for Astarion too!