So I haven't read through this thread and I'll just assume no one else mentioned these issues or came up with these ideas.
FIRST IDEA:
I like to picture in my head that Gen 9 will play as part 2 of a Pokemon duology series like they did with OS where it had both Kanto and Johto featured. The fact that it's just called "Pocket Monsters" in Japan gives me this indication. If they were to make Gen 9 to last for 4 years (to make Gen 10 release on the 30th anniversary), it would give them plenty of time to take the issues and criticisms with part 1 and fix them up in part 2. IMO, Journeys has the opposite issue that SM had. While SM was a bad idea with good execution, JN was a good idea with bad execution. It was stated in an interview that JN is now "Pokemon in the human world" instead of "humans in the Pokemon world" as it has been before. They need to switch that back. Another issue was Ash being set off to the side at times in favor of Goh. This wasn't as big of a deal in other series like DP since it was obvious some episodes wouldn't have Ash do anything, like for Dawn's contests, but since JN is the series where anything goes, he could've participated more. This co-protagonist deal isn't working out. There's also the issue where important characters return for only one episode and barely anything of significance happens or is too rushed like with the Alola group, Mewtwo, Iris, and Oak's lab.
As far as the whole "Ash changing his feelings about Leon" thing, maybe he realizes that he's not ready to compete against him yet, perhaps after not making it to the Masters Eight. We're not sure exactly sure how the PWC works entirely, but I like to think that you need to become one of the Masters Eight by winning enough battles by a certain time in each PWC cycle before it restarts. They could have Ash not make it in time, but then have him do something exciting enough to end part 1 on a high note, such as the Galarian Star Tournament. This would lead Ash into realizing that in order to make it into the Masters Eight, he should do more battling outside of the PWC. Part 2, assuming it would take 4 years, could have Ash participate in a number of events such as the Sinnoh Battle Frontier, defending his champion title in Alola, or even rechallenging the Indigo League and have him win an established league for the first time. They can also have non-tournament mini arcs like the Mohn storyline and the Crown Tundra.
In addition to a second PWC cycle, they could have Ash either use some of his old Pokemon compete in battles, or let some of his captures hang out at the lab like Dragonite, Gengar, and Sirfetch'd so he can catch some more Pokemon to catch and rotate with. It'd be similar to the Battle Frontier where he left Torkoal and Glaile and evolved Phanpy and caught Aipom. He could keep Lucario on him since he's pretty much a second mascot and maybe Dracovish if they decide to have the Gen 8 fossils have their original forms revealed, who knows.
OR
SECOND IDEA:
They have Gen 9 go back to the usual 8 gym battles and Pokemon League format, but this time they could have him reattempt a proper team that he can rotate with evenly. They could have him catch 5 Gen 9 Pokemon as well as 3 Galar Pokemon. That way, he can end up having a minimum of 5 each from Gens 8 and 9 like they did with Hoenn, Kalos, and Alola so none of the generations go shorthanded.