I also wish the game differentiated things more between the 7k spawn living or dying (and I wish them living wasn't such a clean ending, realistically that should have had more consequences), but IIRC, Astarion gets upset at the player if they're killed and stresses that they should have had the opportunity to make their own choices that he received. So he does take away the same lesson that he does if they're saved. And mercy killing them is significantly different than sacrificing their souls to the hells for eternal torment.
I agree with power by bloody means or showing the kindness he was never shown, but I also think Astarion's climactic choice has a layer of continuing to do what he thinks he needs to do, versus finally accepting he can say no. That might sound kind of insane given how much he wants the Rite, but consider - he doesn't want to kill his siblings and he doesn't want to kill the spawn (beyond the knee-jerk 'if I erase them, it's like my shame never happened.')
If you romance Astarion, you're shown how he pursues what he thinks he needs (security) by means he doesn't want to put himself through, but thinks he has no choice, practically. The ends justify the means, even if the means are slowly killing him inside. Imo, once the game explains the first half of his romance, and he becomes genuine with the player - the game repeats itself, and wants the player to catch onto that. Hence he starts manipulating the player again, and putting on a front, and going through all that maladaptive Act 1 behavior all the way up until Cazador's death. He latches onto the Rite as something that will fix everything that's wrong with his life - his limitations, his hunger, his powerlessness, his constant unending fear. He has to do it, or he'll be trapped forever. Or... he doesn't, and can face an unknown future without cutting his own heart out.
If you want my take on redemption... I don't like the way it's brought up with Ulma, because he really didn't have a choice for that. But Astarion does have things to atone for, because... he starts out the game as a horrible person who believes horrible things and will do horrible things given the chance. He had no agency under Cazador, but he uses that to hide from responsibility now that he does have agency, and he gradually grew to resent everyone and everything and modeled his idea of success on Cazador, however unconsciously. When he joins up, he wants to watch the world burn for entertainment. You can't judge him for what he did on pain of unspeakable torture, but the problem is that he rationalized it - in order to cope with his situation, sure, but it did change him. He hurts people because it makes him feel strong, lets him differentiate himself from 'the weak' he's terrified of being, and he uses them for his benefit. Gradually he can realize the latter and then the former are both wrong - ergo, redemption.
I don't really have an opinion on the canon status of his artbook backstory. I think I lean away from it, though I'd like it on a less extreme level (ex, Cazador had bought out Astarion, but Astarion was not specifically aiming to become a vampire... which is an odd aspiration for a very young elf anyway.) Astarion being in league with Cazador before he died is fascinating to explore, and it's implied when the player can point out it's weird that Cazador just happened to be there when Astarion died. But it doesn't mesh well with the cycles of abuse theme his story eventually settled on - because in this case, Astarion starts out pretty horribly evil. I vibe with him being a corrupt magistrate who only cared about himself and fucked people over with his rulings, but when you have him wanting to be a vampire from the outset, and intentionally trafficking prisoners/using the courts to condemn innocents Cazador wants to get his hands on? That turns what happened to him into a twisted form of karma. And I personally find that very uncomfortable to imply given the scope of what he went through. I like the story of a shitty person becoming less shitty through the power of empathy, I don't like human trafficking from the outset.
I didn't know there's a new artbook, though! I'd love to see it, and I hope it clarifies the backstory too.
