Yeah, Astarian wants to become the strongest vampire lord in Faerun and it will be difficult to turn him from that path.
I really don't think so. He said he just wants to kill his master and be free. I know he can lie but his dialogues are quite consistent in this matter.
Also, there is this moment in the game
when we can talk to the magic mirror as any party member we want. The mirror asks the character what he would like to see there the most. Astarion has 3 options: to see his real home he haven't seen for centuries, to see how to control the tadpole and to see Cazador burning in the sun. No great power or becoming a vampire lord.
I think his greatest dream is just to be free (obviously). Maybe later he'd like to become an important person in the city once again or something, but to be honest I didn't find any evidence in the game for him wanting to be a vampire lord. On the contrary even. I think it's natural to assume he wants to be an evil powerful vampire because he is an evil character and a vampire slave, but his plot doesn't seem to go there, at least for now. Who knows, maybe he can change his mind in Act 2 - or maybe PC can change his mind - but for now I'm pretty sure he doesn't want it.
Of course he could do it if it were the only way to break free from Cazador.
