Damerdal
[Dam]n h[er] Gan[dal]f!
well everyone knows I don't like Lillie, the league and a lot of SM stuff, but it's already happened. Ash is in love with Alola, he is the champion of the region and his adventures here were significant (He got the Z-Move, the champion title, a father, friends, family etc.) The amount of Cope on SM only shows that there are more childish people than "SM Ash" himself
Agreed, in other regions, Ash was always traveling. In Alola, he pretty much has a second home and school. It's obvious he would be more attached to Alola, on top of the other things you mentioned.
As for the discussion on Kukui, I don't know if I'd go so far as to rate him on the level of other regional Champions like Cynthia and Lance. However, I think there's a strong implication based on how he is presented, as well as the out-of-universe reason of him being a Champion in the video games, that Kukui is definitely an Elite 4 quality trainer. By beating Kukui, Ash is narratively shown to deserve the title of Champion of Alola, tantamount to the other regional Champions. It makes sense in the overall progression that in JN, Ash naturally rises from being a Champion to the outright Monarch.