My Dark Urge always gets a nervous eye twitch when the ascended Astarion speaks and I think about the Slayer or Powerword Kill. I just want to romance Astarion because he's just extremely funny... in his own way. But I can't. Still, I have problems with both the spawn and the ascended Astarion. For example, I don't want to start the romance because he's grateful to me because I won't let him bite the blood merchant. I don't want him to thank me for that. The other way is also difficult because I don't want him to feel bad.
I myself had a hard time accepting some aspects of Ascended Astarion's behavior at first (really don't see where he's "grumpy and boring", honestly). And it was frustrating to the point that I think it was quite appropriate to add the line "Sob" for Tav, after the famous "On your knees, dear" :)) After all, that's what I successfully did in real life, after the first time I got into that scene :))
But try to take a deeper look at the situation. Remember what Astarion himself had endured over two hundred years of torture, humiliation, and "pure shit," as he put it. What was required of Tav now was nothing compared to that. After all, what's more important, pride (or someones own idea of pride) or the happiness of the person you love? I'm sure that if Keterik Torm could have gotten his beloved back through even worse and more horrible humiliation than what Tav has to endure in this scene, he would have done it without hesitation. And it wouldn't have stopped him from being Keterik Torm. Tav, too, by "submitting to Astarion" ( just to make him feel more comfortable) loses nothing in "coolness". And to understand what's at stake, you can imagine another way - Astarion= Spawn, I won't write about what's on this way, so as not to repeat what has already been said many times.
Perhaps I, as a player who chose the "evil path" solely for Astarion's sake will have a slightly different feel for the game than you, since you chose this path for the sake of the path itself, to be Baal's Chosen One and enjoy it. But, if you love Astarion, try to understand him, feel for him, put yourself in his shoes, maybe you yourself will want to give him something that will calm him down, allow him to trust you and make him at least a little happier. If that's not acceptable in any way, then maybe it's best to remain just friends with him, but still help him ascended? Especially if you are on the "evil path" yourself, it would be unfair not to give Astarion the opportunity to become powerful, free, and independent as well.