I am also in the camp of "your party probably talks to one another off-screen". In other games I always thought it was incredibly artificial that you could, say, leave your good hearted heroic companion at camp, walk off and murder a woman in front of her child, then go talk to him again and absolutely nothing will have changed.
Like, no. Realistically people talk. Your companions aren't taking a vow of silence every time you go off to do something. And it feels like a cheat in the system that you can act counter to one of your companion's alignments by just leaving them at home when you go out and rob a family blind because you need some extra gold for a new pair of magic boots.
As far as your specific problem? Astarion would find your character incredibly boring. A stick in the mud who doesn't let him have any fun. Someone who sees the world in a completely alien way to him that he can't understand or really relate to. How would you romance someone who you have nothing in common with and who finds you dreadfully bad company?
Your choices aren't just things your character does. They also inform the game what kind of person your character is, which informs your companions what kind of person they've been dealing with all this time.
Basically, if you want to romance a guy like Astarion you're gonna have to get your hands bloody and act in a way that Astarion finds interesting and delightful. Keeping him at camp to try and hide who you are from him wouldn't work out.
So... Just be yourself. He has to love you for who you are on the inside.
