So, and this is my own personal headcannon, but I don't think it's possible for someone to change in so short a time. I don't think he changes no matter what path you take with the ascension.
I agree that he's not the way he is because of his mortal life, just that that backstory shows he started off as a bad guy and Cazador made him worse.
And tbf, I haven't done a personal non-ascended ending myself, but watching the videos of others, plus my own personal Ascended version, Astarion tells Tav exactly what they want to hear in both endings. As long as you don't break up with him.
If you don't ascend him, he tells you that it'll make him a better person and that Tav has made him a better person. Which is exactly what a manipulator will tell someone they are still trying to manipulate. Especially someone who is powerful and who he wants to keep on his side and who has been trying to nudge him into being a better person along the way. "I wasn't a good person, Cazador made me do bad things for 200 years, but a few weeks with you and I want to be better/am better for it." I just don't quite buy it. I think going back to being a weak vampire spawn makes him more fearful than ever because he can't protect himself as well as he could with the tadpole, so he has to go back to manipulating Tav to stay on his side, like he did when he first seduces Tav.
If you ascend him, he just doesn't have to hide what he's actually doing/thinking anymore. He can be open and more honest about his craving for power, especially since most Tav's that get this ending have been helping him power trip along the way. Now, a lot of people say "Well true vampires in DND don't have a soul, so the Ascension makes him evil." But the Ascention doesn't really make him a true vampire. He'd have had to drink Cazador's blood to do that and he doesn't in either scenario. However, Raphael states that he would be a 'living' vampire. With all the hungers of a mortal man. So Ascension doesn't really make him a true vampire but something new. Something that may have a soul but having a soul doesn't suddenly make someone good.
Which is probably why I'll always Ascend him. His extra damage is nice, being a vampire myself is fun and the 'good' ending still has you either killing 7000 people or releasing 7000 vampires with no control and a whole lot of hate free. Yes, it's into the underdark, but how many ways could we get to the underdark in just Act 1 alone? Way too many to be policed forever. Not to mention that if you get the tavern scene with Astarion's siblings, one mentions having a human waiting for him to murder after the ascension when he still think's Cazador is gonna free him. So, they're not exactly the good guys either. There's no way to know what those siblings will do with an army of 7000 under their power.
So I'll just save myself the trouble of redemption and power trip out.