To me I think the main argument would be that going down this path means he's always going to be in a cycle of never being satisfied with the power he has and always seeking out more power for the sake of never being vulnerable to anything (because there's always a bigger fish, a group of high level adventurers, a devil or demon or angel, etc that could threaten him). But I recognize that argument isn't the strongest.
Although you say it isn't the strongest argument, I think the text in the game agrees with you about this, honestly! It seems to be part of AA's characterisation, and it's super fascinating and interesting to me. I'm going to go into a bit of a tangent here, so I hope it's still also interesting to you even if you don't like Astarion:
The strongest indication that points towards what you're saying is this devnote: https://imgur.com/a/op0EWKa
The rest, I think, is puzzled together through the setup of Cazador's palace and by the full picture the contrast between the two routes paints.
Cazador's palace introduces us to the very deliberate story of how Cazador used to be in Astarion's place under a cruel Lord (who, in turn, also used to be a spawn, etc etc), but through a ritual gained power and access to hedonism and riches. It still wasn't enough for him though, as seen with his pursue of Ascendancy and slavery of other individuals. Where this gets interesting for me is when you kill Cazador without Astarion and use Detect Thoughts on him, which gives way to one of my favorite monologues in the game:
"These deathless dreams hold memories of a mortal life once-forgotten. Of the boy I was, the man I became, the monster that will not end. I sleep, but cannot rest. I live, but cannot die. I am eternal, and I grieve."
I think what's noteworthy here is that none of his grievances here would actually be solved by the Ascendancy he seeks, unless it does something to his sleep, which doesn't seem to be the point here tbh. These grievances seem to rather be the product of this mindset he's gained after everything he's gone through, and his life has become rather empty and meaningless in turn. He is in a way trapped by what's happened to him, and has to cope however he can manage to. This exploration of his psyche, given the other quite deliberate moments where we're informed this is something that keeps happening and the parallels between Cazador and Astarion, seems to be foreshadowing the mindset AA will eventually be in.
Now, as for the contrast in routes: There are a bunch of interesting things here, for me. While the Spawn route already pieces all this together by what he says about, amongst other things, the "cycle of power and terror", this is a point of contention that's quite often debated so I'm not going to talk extensively about it, though I will plainly state I think the narrative isn't trying to trick anyone here and he's being sincere, I feel like the story's informing you of it's themes and arcs here quite unsubtly (to my dislike, honestly). The two things that come to mind (it's a shame I'm not in my A game right now because I /know/ I have more examples I can't recall right now that I'd love to talk about but I'm quite sleepy!) is the contrast between the routes regarding what happens to Cazador's palace and a couple things in the epilogue.
So, AA lives in Cazador's palace, now his, I suppose. It's a very literal way of expressing he's stuck in a traumatic place. He's routinely walking the rooms where he used to be whipped, tortured, sexually and psychologically abused, etc etc. And this is where he chooses to live. That's fascinating to me. Meanwhile, Spawn burns it to the ground, which is pretty straightforward in how these two routes inform each other.
In regards to the epilogue, AA talks about being lonely (which is another aspect of what I think the ritual does psychologically to him, in how it reinforces his view of trusting no one and having to be on top. Idk if I'm reaching with this one but I also think that here, the previous tadpole dialogue where we're informed his worst fear is "Complete solitude, being voiceless" and similar but less specific cues that he longs for understanding and company are a very cool thing to consider) but, most interestingly to me, he remarks how he's not even out in the sun much, because he'd rather be scheming in the palace. So, despite everything, he's not even truly enjoying arguably the most important boon the ritual gave him, because he'd rather be doing some classic paranoid power-hungry vampire lord stuff. What this means is that either 1. He was lying from the get-go (maybe even to himself) and didn't truly care about the sun that much or 2. What this whole ritual endeavor has meant for him has severely changed his priorities so that specific flavor of freedom he initially sought is kind of secondary now to powerplays in the dark, again, in the palace where he was a slave for 200 years. That kind of pattern, I think, leads to bitterness. Basically, AA lives hedonistically, but psychologically, I don't really think he's happy, besides surface, sensory pleasure. I'd go more into this but I think I'd go even further into my-opinion-territory which is bound to arise some debate and I don't really want to discuss those all that much.
Finally, since I've been growing increasingly anxious about discussing this stuff here, I'm going to ask people to either engage in good faith or not at all. I'm super passionate about (further) corruption arcs and tragedies/tragic characters, so to me AA is very interesting and I wanted to engage with someone who holds a similar opinion here. I don't seek to convince anyone or change their minds, just to talk to a likeminded person on the thread, so feel free to ignore if this is the opposite of how you view AA.
