I've read through this thread but forgive me if I'm mentioning things that have come up before. I just finished the game with a romanced Ascended Astarion and a few things came to mind.
1. During the end of the final battle, Astarion made it very clear he thought Tav should control the brain, and obviously I chose to destroy it, but there's absolutely no mention of this from him in any of the ensuing conversations. I don't think he should break up with Tav over it (partly because I think he knows as well as Tav that the brain can't be trusted, even when it seems to be controlled. And partly because it would obviously break every conversation you have with him after). But I do think he should have some reaction. It could be as simple as an offhand comment, which could be either condescending ("It's a shame your ambition faltered at the last minute, but no matter.") or kinder ("I know I was determined that our path to taking Baldur's Gate would be through the brain, but you were right to be more level headed than me.") And that could lead really well into the "What comes next" question. It could be something he says unprompted, or there could be a dialog option in the first part of the bedroom conversation for Tav to say something like, "I'm sorry I chose to destroy the brain. I think there are better ways for us to grow our power."
It's just really odd because the nature of their relationship at this point is that Tav is meant to be "wonderfully obedient" and they basically disobeyed a direct order, or at least a very strong suggestion, and it's something Astarion has made clear that he's wanted even before he learned of the Rite of Ascension. He should acknowledge that it didn't happen, however he feels about the result. In fact the first time I played the ending I chose to control the brain, wondering if he would break up with me if I didn't, and when the game cut to credits I knew that meant he doesn't break up with Tav, since I was aware of the epilogue party and all that.
2. After turning Tav, Astarion mentions that their vampirism is probably muted because of the tadpole, and removing it will likely result in further changes. He acknowledges as much if you try to break up with him, saying that now that the tadpole is gone, you're pretty much stuck with him. But if you don't try to do that, there's no acknowledgment at any point, from him, from the narrator, or from other characters observing you, of any outward or internal changes. I don't know precisely what those changes are meant to be, but I'm assuming that at the very least, Tav's eyes will turn red and they'll become paler and grow fangs (it's weird that they haven't already, but I get that that would probably interfere with the player wanting to use the magic mirror). Maybe their bloodlust starts to kick in. Maybe there's simply some mention from the narrator during the dock scene that when you look at Astarion now, you feel some strange connection, not unlike with the tadpole, but only for him. I don't know, but Tav has fundamentally changed and it would be nice to see that acknowledged, the way it is for other characters who've undergone physical changes, like mindflayer Karlach.