Of course, Astarion wants to ascend. He wants it so much, it’s as clear as day. He has never hidden how much he ideally likes the idea of power—to elevate himself from his current position, to ensure his safety, to bend others to his will (instead of being the one who is bent). And if he can also walk in the sun and never feel the hunger pangs again, even better!
But let’s not forget that Astarion has a limited worldview. Cazador himself says it in one of his confrontations with Tav/Durge:
"He is afraid. He is afraid because all he has ever known is you and me, and without us, he is nothing."
Astarion does not have a well-developed sense of self, and by default, he also lacks many of the skills that a well-adjusted adult should possess. So, to navigate life, he can either rely on the worldview presented by Cazador (power, power, power, and more power—to place himself above others) or the one offered by Tav/Durge, assuming they are a heroic figure. Otherwise, the only perspective left is that of power, and Ascending becomes almost natural in an evil playthrough (which I myself did in my villain run). Ascending Astarion in a good playthrough, however, seems completely contradictory to me, but whatever…
Let’s not forget that power is not Astarion’s driving force—power is only a means to an end. His real driving force is fear, as both Cazador and Scleritas emphasize. He would do anything to feel safe (like becoming a half-Illithid if scared enough by Tav/Durge—even though he rejects that idea with every fiber of his being, and yet…). The scene with the dryad, Naoise Nallinto, in Astarion’s origin run makes it crystal clear: when she uses her power on him, among all the possible choices (wealth, respect, power, etc.), Astarion’s personal wish is to feel safe, not power—even though power is explicitly one of the options. But it’s not his!
Oh, and Astarion himself spells it out, right before the final decision between Ascending or not. His exact words:
"One final thrust, and I'll be free of you. I will never have to fear you again. And if I complete the ritual you started, I'll never have to fear anyone. Ever."
Everything revolves around fear, which is once again emphasized in the insight check—where it becomes obvious what is driving him and what is simultaneously holding him back from making a rational decision. Because while it’s true that he wants to ascend, he also wants to redeem himself. Well yes, it's shocking, folks, but two completely opposite desires can exist within the same person. They're called internal contradictions, and we all experience them every day or almost ("Oh, damn, I want to go out with my friends tonight, but I also want to just lie on the couch and watch TV").
Let’s not pretend this character is one-dimensional and that all these dialogue lines don’t exist when discussing Astarion. Of course he wants to ascend—he wants it so badly. The point is understanding why he wants it. And then questioning whether giving in to that fear is truly worth it, considering the consequences and what he would be giving up (because even Ascending comes with its own sacrifices, and I’m not even talking about his soul or the 7,000 people).
That’s why, if they choose to, Tav/Durge can intervene and make him reflect on the alternative (which, depending on how you play your Tav, could have been introduced to him from the very beginning of the adventure—it’s not something that just comes out of nowhere, unless you’re playing completely incoherently).
And it’s Astarion himself, in one of the most beautiful dialogues in the entire game, who explicitly states this lesson he has learned. When Durge is overcome with despair and fear—just like him—and tries to end the relationship, Astarion says:
"This little adventure of ours has taught me that we can't let our lives be ruled by fear, or else we'll never truly live."
He has understood. He has grown. He has accepted that uncomfortable emotion and has decided not to be consumed by it—to choose for himself without letting fear dictate his actions. And I couldn’t be prouder of him.
One last thing, because I’ve seen it repeated a lot on social media:
Ascending is not Astarion’s lifelong dream—it is Cazador’s dream. Astarion didn’t even know this kind of ritual existed until five minutes before it happened, so no, Tav/Durge is not cruelly ripping away his lifelong dream just for the sake of moral superiority. And above all, they are not forcing him to give it up—but I’ve already talked about this before, and I’m not going to repeat myself.