Agreed — I was worried the prequel would be a bit of a money grab follow-up on the first one's popularity, but (1) it's a standalone story which is great in its own right, and (2) in the few places where it does have some narrative connections with the original, it is done very well, and tragic/bittersweet.
Yes, I am always worried about prequels for that reason, but I think I enjoyed this one more than the original. They are really very different in scope and story as you said, and the bittersweet quality of the connection between the two books enhances the ending of the prequel in my view.