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.