I completely agree with everything you just said. The character development is the only highlight. It just feels like a sad excuse for the first trilogy, but without the originality and excitement.
Also, my own personal pet peeve, I HATED Nina's character arc. It just felt like a betrayal of Matthias. The beginning, where she was hearing his voice, could have gone somewhere cool. But then they didn't do anything with it and instead decided to give her another love story. Despite the character chemistry there, it just doesn't feel the same as Nina/Matthias. It makes me sad.
There was my rant. They were still good, just not comparable to the others.