I don't even romance Spawn Astarion because I'm not super interested in that (did it once to see it but not my thing), but "Bad boy that makes exception for you" is clearly not what he is lol. Just look at the friendship epilogue. Or really, the fact that you can make him see the ritual is not truly what he needs even if you're friends or you don't even get along. Or the differences in dialogue between Spawn and Ascended during the final boss fight and ending in regards to other characters that aren't your PC.

Astarion wants power, yes, but the reasons he wants it for clearly are 1. self-hatred (he thinks who he is isn't enough and is too pathetic/weak) 2. fear. There are 3 moments I can think of where he expresses doubt or concedes your points if you're trying to dissuade him from the ritual. I don't think we should act like his hunger for power is his true, unaltered, pure desire unattached from everything that's happened to him and not mostly derived from his insecurities and trauma, as is unravelled throughout the game. His past as a magistrate is not playing much of a role here, I think, especially as the story basically doesn't even bring it up past a couple nods, as much as I wish it did.

I don't think it's a stretch to see that Astarion's good arc starts developing quite early on and is not sudden, as he drops his facade and barriers when he lets you read his scars, opens up to you about his consent issues in act 2, etc... do I think it's more interesting to corrupt him even further and make him regress? Yes. But I definitely got the impression that's where his arc was going even in my first blind run, pretty basic "what he thinks he wants is not what he really needs" arc. Saying he's petting bunnies is also disingenuous, I don't get the derision for a version of Astarion that's Chaotic Neutral at best.