If you romance him while roleplaying an evil character then I doubt he would have a redemption arc. But if you romance him while roleplaying a good character I don't see why he couldn't have a redemption arc. People aren't static. They can change depending on life events, people they meet, etc.
At least because you need to do evil things to get Astarion's approval. If you do what he doesn't like (good actions), you won't get approval and you won't have romance... So how he will change if you need to do evil actions?
So if he can change in other way, like ‘not approved’ way, it’s stupid. Cuz Wyll, for example, doesn't get evil, he just walks away from you if he don’t like your choice. This is logical.
Pretty sure, if you go ahead and tease him a little it doesn't matter. I definitely romanced him on neutral and mid approval.
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...