While I do think SM Ash's development was occasionally slapdash due to experimentation, I think it did work in terms of seeing a new light to him. Ash has never been forced to sit down and just appreciate the backdrop before, he's always been linear focused on his goal and just running to the next step in it. There was a real fish out of water take to Ash because he had to redesign his whole approach in Alola. The same happened with Team Rocket to some level.
Comfort zones seem to actually be a recurrent theme in SM as well as family, like most of the characters seem very used to going about things a very set and limited way and now have to branch out, which kinda works for characters who don't even travel much in this series.
Concerning the companions, I think generally their secondaries served as their 'bookend', the pivot that starts their development out of their comfort zone.
Kiawe for example demonstrates his inability to be versatile outside using Turtonator's tank approach and his Fireum Z move for everything, as observed with his Charizard's inactivity, his epic fail in the Pancake Race or the switch up episode where Popplio has to make the first step to reach out to him. Marowak appears, defeats Turtonator and crushes that approach, leading Kiawe to train a more versatile battle style for Turtonator and capture Marowak, who has a much more head on style. We keep getting little details as the series goes on that show Kiawe is trying to branch out, learning to trust Marowak and teach him Inferno Overdrive, unretiring Charizard after he (rather than Ash) impresses Tapu Fini, collecting a new Z Move and ultimately reaching the semi finals in the league with all three of his Pokemon used to their fullest potential.
Sophocles meanwhile starts off a rather sheltered and less sociable character, this culminating with the moving away mixup and him thinking everyone really hates his guts. However this is disproven and he gets Charjabug as a gift from his friends. Kiawe and Ash spend the whole series helping him bulk up Charjabug as a personal project, developing on his tech niche, giving him a personal rival and ultimately helping him evolve Vikavolt. Elements like his phobia (which is introduced very early via Togedemaru) against his desire to be an astronaut are pieced together nicely via Vikavolt helping him get over them, and by the end of the series Sophocles has started training independently, teaching both his Pokemon new moves, mastering Bugium Z and making it into the quarter finals of the league.
I think the key problem here is that while Kiawe and Sophocles got Marowak and Charjabug pretty early in and thus had plenty of time to slowly bulk up these small accomplishments and details about them that all pieced together nicely, the girls all got their secondaries near the end of the series. Shaymin for example bookends Mallow's nearest attempts to a character arc, similar small attempts to be more daring and less unsure and coddling, but Shaymin and Mallow's background and introspective didn't appear until some way into Ultra Legends, as said, there was barely anything building up to it beforehand, meaning she could only scrape up so many tidbits of development within a very small amount of time. She didn't have the time to be a slow burner.