And it's still something. You still need to do something for it. My opinion, Astarion should not approve of good choice. He literally approves of kicking squirrels and killing anyone lol. So if you choose a good option and he makes that pained voice, it should cause him to disapprove of your choice. This is what happens with characters, well maybe not Gale, he only thinks about magic food. And I don't see why it should be any different with Astarion. Like I said, Yes, maybe you can make him neutral. But kind? ... meh...
Also happy ending is not equal to a "good" ending, if you know what I mean...
edit: I mean imagine that you are playing as evil character and good character changes to the evil side (this is the reverse example). I've never seen this before... It's not fair that ppl always trying change evil characters, but this never happens with good characters.
I feel like If we took Astarion's word at face value, he would never share his power and freedom once he gets it. That dialogue line says, that Astarion will be willing to do it (whatever it is). Maybe you even lock-in the loyalty of your party at that point (I mean Larian mostly said this, but I am not sure how much it can be taken as in they would never betray you), maybe he will not want to side with the hero path, if our approval is below certain point. The nature of tadpole and who tampered with it might give him enough of a reason to try and learn more about it and use it's power. But yes, I see him ditching on the player character if we somehow get rid of the tadpole before resolving the main quest
As I mentioned in one of the previous posts - I believe that even character like him could go through with the 'hero' path, even if the path that gets us there is very grey. I personally don't want him to change his opinions or be more kind. I like that he is snarky, nasty, kinky and all over the place.
As for the good ending or potential cure to vampirism: This is an idea that sprouted from Astarion enjoying the sun. Nothing else.
If there is some clash of gods though - and you do decide to side with the good guys - maybe there would be more of a reason for him wanting to reverse is, have his soul back, especially if the tadpole is gone and he would miss the sun too much including the player character
I think there are doable ways to make 'evil' characters make choices they wouldn't.
