The animal bit, I'm torn about- on one hand, it could be an actual moral backbone on his part, or it might be the residual fear of Cazador. Cazador never allowed Astarion to drink from humans, only rats, and in some dialogue it's implied that he feared that Cazador's old orders still had sway over him.
Without going into too many datamined spoilers, it's not moral backbone, it's all about Cazador's control and breaking free. If he promises to feed only on your enemies, or only on animals, he seems to keep that promise, but I doubt he'd be limiting himself on his own beyond, "Don't piss off people with pitchforks!"
I'm on board with the Red Prince comparisons, but if Astarion's romance also
ends with us becoming his sidepiece, I may turn him over to the Gur.