I liked the phrase "nothing special, of course" at first, it sounded kind of touching and sweet, but... There is a sense of uncertainty, and the cough at the end is unsure, as if he doubts whether he said it right, whether he should have said it. It's as if he's starting to "watch his words", and I want Astarion to be natural, to be himself, to say whatever comes into his head without hesitation, not to restrain himself in any way....
He was never fond of sincere public displays of affection. When he wants to talk about his feelings, he usually takes the player character aside, while the things he does in public are at least slightly performative. (His new kiss for example is incredibly performative. I always employ it after fights, when everyone is covered in blood.) The "you are only the first person I truly care about" is probably a little bit more sincere than he is strictly comfortable with in public so he drabs it in irony and still needs that little cough at the end - as if he had just said something indecent.
And for me the morning conversation (I was referring to the one after the graveyard, not the one after the brain) wasn't sweet at all, it was sincere. He isn't "happy" but he feels that he made the right decision, that is neither surprising nor sad for me. It's expected. He gets to / has to decide who he wants to be after all. That is serious business. He is allowed to be serious, sincere and none-performative for a bit. Depending on what you choose during the Post-Brain-Dialogue, he is already more confident then. During the six month until the epilogue and he has time to find himself and he is more honestly happy then.
Otoh, Ascended Astarion knows who he is, he is the person who Cazador made him to be. Everything he is, he is because of Cazador, because of Velioth and the whole line before them. Killing their former master and taking over from them seems to be how vampire-inheritance works. There is no room or necessity for doubt but there is no freedom in it either. It's bowing one's head to fate, submitting to destiny. For me, that is sad.
You could say Ascended Astarion really isn't a "new kind of vampire" he just is an especially strong kind of vampire. Spawn Astarion, though, is a new kind of vampire because he resisted the call of ultimate power and taking over his inheritance - which, from what Astarion tells us in Act 1, is unheard of for a vampire.
It probably depends on where you place your priorities on the axis of freedom vs security and destiny vs self determination.
As for the Underdark: He already got everyone out, I think the other spawn can take care of the rest. But it does explain why the Underdark ending has a much more somber and dark mood than all of his other epilogues for Spawn-Astarion. He even sits around the fire worrying if the other spawn are alright. Heavy is the head that wears the crown, I guess. He is a ruler in that ending, but a responsible one, who cares and endlessly worries about his subjects.
