You’re probably right. He had great inspiration for the first 3. All downhill after that.
Heck, even ROTJ had some dumb problems with it (not as egregious as the prequels), but making Leia Luke’s sister was problem #1. Making Darth Vader Anakin Skywalker was the other problem. It completely turns Obi-Wan Kenobi into an abject liar.
Ewoks could have been better. This article explains things very well....
http://www.filmthreat.com/features/172/
However, there are a few things about ROTJ I like very much, but it doesn’t save it.