I'm currently doing a Durge evil playthrough with Ascended Astarion as my romance and...it's crazy how much more romantic interaction there is.
I mean, it's dysfunctional and manipulative and clearly not the "good" romance he or your Tav deserves but there was a lot more effort put into the interactions you have with Ascended Astarion than spawn Astarion which...makes no sense.
I still don't know what the ending is like; I'm bracing myself for it to fall flat like the other ending. But Astarion is already talking about our future and plans for us and when he was "good" spawn Astarion, I got literally no interaction or discussion about that at all.
Getting more and more annoyed that I'm enjoying douche-bag Astarion more....
There is quite a lot of romantic interaction for a "good" DUrge, too, though. It has even one of the sweetest and most tear jerking interactions with him -
after you defeated Cazador you can try to break up with him, saying that you are afraid you might kill him one day. And his reaction is spot on. One of his VAs best performances IMO.
Spawn Astarion needs some work, but I'd never choose Ascendant over him again.
But you don't have to be nervous. Ascendant ending does not fall flat like the spawn one does.