Again, this is why Gary Kurtz was purged from Star Wars so that we can never have a perfect trilogy and have thirty-five years and counting of unrelenting misery.
According to the relevant information available, Kurtz was fired already during ESB because he didn’t manage to do his job as a producer. He let principal photography to go way over schedule and way over budget (something that is unacceptable for a big studio production, let alone self-financed production). Even if he stayed things wouldn’t be much different. Lucas always had it his way. Even in ANH since studio didn’t seem to care.
Convenient excuse; Lucas was jealous that Kurtz made a better film than he ever could. Empire is the best because Lucas has almost zero involvement, and conversely Jedi is the weakest. The Ewoks and twincest were all George’s idea.
You can easily find the information about ESB principal photography time and budget increase in several sources. Also you may not be aware, but Kurtz was effectively replaced already during the ESB principal photography (far from the final product) so your “jealousy” theory is rather silly.
Zero involvement? The only part he wasn’t involved was the principal photography, which was Kershner’s domain (the main reason he was hired for obviously).