Kurtz was fired because of Lucas’s ego, and the budget issues on ESB were just an excuse. Lucas could not stand someone else making a better SW movie than him, one that put story first and merchandising second. If Kurtz worked on Jedi it would have blown Empire out of the water.
The ending would have been way better but as it pertains to the totality of ROTJ it is uncertain. Surely things like Luke skipping the end of his training and the Ewoks being so focal wouldn’t have been written into the script but Kurtz’s version of ROTJ would need to do more than fixing the mistakes of the original ROTJ to be legitimately better.
The ending would have been the same. In ESB, Kurtz was not even involved in story creation to begin with. As Kasdan stated about the draft Lucas written “The structure of the story was all there”. If Lucas wanted it to end that way, Kurtz could do absolutely nothing about it.
From what I heard Kurtz’s ending was bittersweet with Luke going into exile and Han dying. Though the latter could have reportedly happened earlier in the film.
It is completely irrelevant since he had no control over the story.