I would actually say that in broad strokes, JN plus MPM does provide a great framework for Ash's journey ending: partaking in a global tournament and facing a trainer deemed the strongest of the world and eventually realizing what a Pokémon Master is roughly how I would've wrapped his storyline, and I also believe an open ending of Ash heading out for his next adventure is the most fitting closure for the kind of character Ash is. I am however not the biggest fan of the series's execution of it, and I have my doubts about if JN was even initially conceived as Ash's final series until the halfway point or so.

That said, my personal preference would probably lean towards making understanding what a Pokémon Master a core part of his arc rather than something asked and addressed in the final episode only, something to give an emotional center to his final series. Probably have a check in or reference to all past companions and other major characters to give us an idea of how everyone has grown, and probably leave the door open for future challenges and adventures in a vaguer way than the Anime did in MPM's final episode: I feel by the end of MPM the series closed a few too many doors to leave Ash doing more than basically just 'Pokémon of the week' encounters going forward, and it's part of why I feel the implications of reunions and new encounters of JN's proper finale hit the same notes a bit better for me.