I also think you haven't seen the true Astarion. You all judge so quickly on all these characters. The way I see him, and the little hints I've caught, he's messing with you all. He's making you think he's just some subservient, flamboyant victim.
To be fair, you are also building an entire scaffolding of judgement and character assessment here all on your own with virtually nothing to support it and a lot to detract from it... so saying others judge quickly is a bit rich, when it seems like you've already made up your mind about who each of the characters are, and decided, quite firmly, that several of them are not what they seem while others are - all based largely on a personal preference and a fiction you've built around them that isn't reflected in the game in any tangible way.
writing him off as someone who is not a threat.
He's not a threat. Any one of my halflings can go toe to toe with him and kill him stone dead, and he doesn't reveal any special mystical super vamp-strength to save his own existence, he just dies. He is simply weak, just like the rest of us are at this stage.
If he had truly wanted to kill you, he could have.
No, he couldn't, because on several occasions he was goaded into trying, and he failed, spectacularly, to his own swift and forgettable demise.
The MC assumes he's just a noble idiot and winds up with a dagger at the throat.
Some do, some don't. Some catch him in the act of trying to surprise you, because, at the end of the day, for all his effort to do so, he's not really very good at it, at all... you just have a chance (a less than 50% chance, usually) of being worse at noticing things, than he is at hiding them, and usually you're odds-on for being better than him here.
He just submits over and over again, going along with whatever you decide unless you decide to attack him or betray him, and he continues to act all friendly no matter how much he disapproves of your decisions.
His attitude towards you changes quite substantially depending on how you treat him and how you act, much like the others. In most of my games, for example, he doesn't really act friendly towards me at all, and instead talks like he feels very put upon just interacting with me.
I think he's one of the most powerful origin characters in the party. Use Stealth and cap enemies with Sneak Attack over and over again. He can Stealth just about anywhere at any time.
He's a rogue, and no more or less powerful than any other level 1 rogue. He has nothing at all over any other level 1 rogue, who can also stealth just about anywhere, and pop sneak attack most turns just like he can. None of this is about Astarion, it's just about the rogue class. Nothing to see here.
Unless you are greedy and murderous, he disapproves
Chaotic and bloodthirsty, I'd say... and he never pretends to be otherwise. This isn't an indication that he's secretly powerful, just that he's chaotic and bloodthirsty. This also has nothing to do with his fashion sense, his flamboyance or anything else.
