I'm pretty much in the same boat. The first was interesting. The premise was nuts, but whatever, that's the setting of the story. The second was, as you said, a bit of a rehash, but there seemed to be an honest attempt to explore the world and expand on things and all the good stuff a second movie is supposed to do. I remember that we watched the third one, but I couldn't tell you much about it, and I honestly don't remember if we saw the fourth one. The worldbuilding was getting silly and I wasn't engaged at all. I guess there's a new one out now, the start of a new series or something? Haven't seen that one.