please forgive me if I don't massively quote and respond to every point like I did before, a bit too tired for it right now, but overall, I agree, Astarion is a manipulator. All our current companions are, minus Wyll I'd say.
And yes, unpopular opinion, but I find Shadowheart to be quite manipulative too, not on the lvl Astarion is, but she's close enough. Anyhow, after Anders from DA2 and Solas from DAI, I'm hella paranoid of all possible romance options in games. And with popularity of GoT (like I mentioned in previous thread), I really think we're in for some interesting plot twists. Not necessarily regarding Astarion, but the other companions, especially Shadowheart, are on my radar... and Gale. I've come across this entertaining conspiracy theory on reddit about Gale being truly evil. It's an interesting read for sure:
https://www.reddit.com/r/BaldursGate3/comments/jlu5yh/ruminations_whats_your_crazy_theory/I disagree with most of them being manipulative.
I haven't romanced Shadowheart, but I did reveal her secret and had her in my party for several playthroughs and I do not find her manipulative. Very secretive and likely to bacstab you? Propably, but honestly - all of the characters are. And I really like it because it invites some interesting dynamics and so much potential for gazillion playthroughs.
Lae'Zel is just doing what she thinks is best, I feel like especially after 1st tadpole power she is friendlier, although approval might have triggered that as well

Anyways, she warmed up to me, both of them did. And I don't feel the looming gloom as much as with Astarion.
I've read that theory about Gale! I think it's very likely there's more to him than meets the eye. I must admit I am not very interested in having him around once he ask to eat relics. What if I am feeding some evil god?!
I legit think Astarion is probably the most honest LI in right now
To be honest I thought this up until I went to the hunter without Astarion. I don't entirely believe the reason why the vampire hunter is after him specifically (discussed it somewhere above in spoilers). My main reason is that the hunter doesn't know how he looks, which seems like a major oversight someone as scheming as Cazador sounds.
If he is truthful about that, than I believe that Astarion is indeed the most honest LI. If he is lying about it, than I am really curious what the reason could be.
There is so many questions, as to how the story will develop and it's also core to understanding the possibilities of him backstabbing you... Because of how power hungry and
broken/damaged Astarion is, he shouldn't be completely trusted.
If the tadpole stays in our head the entire time in BG3, than I trust him,
mostly. This would solve the issue of Astarion leaving the party in case you help him with his quest -
whatever that is. Without a tadpole and being
true vampire. I don't see why he would stick with you, as he would likely burn in the sunlight.
He is likely to manipulate you to help him kill Cazador and once he achieved both goals, he is free. He mentions maybe having his own coven in one of the dialogues I believe. He also talks about how it never happens, that a vampire shares in his power with a vampire spawn. I believe he is honest with both of these as his goals.
If we can manage to control the tadpole powers and unless there is some sudden power shift and he feels like he is on the losing side I think we are all good.
Anyways the true bad ending to Astarion romance, would be losing him from your party too soon.
edit: I also don't know why someone think Astarion's romance doesn't need approval. I saw he’s reject someone at a tiefling party.
Pretty sure, if you go ahead and tease him a little it doesn't matter. I definitely romanced him on neutral and mid approval.
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.
My idea of a 'redemption' ark for Astarion is bringing in his morally damaged character to do the right thing... If you can achieve the good ending together, no matter what way you got there. It's still good, only chaotic good

Astarion's internal dialogue clearly says he is proud if he party sides with the dark forces vs.
this.
It is clear what his choice would be...