While I totally agree continuity of Ash's personality has been wonky
I still think it can fit in one timeline consistently, albeit with a lot of guessing and assuming rather then something concrete the anime gave us
After DP and him going all out with every Mon he took a break at Pallet town (he even said during end of DP he didn't know what he was going to do next and after his mons performance at the league he wants to rest in pallet town)
So when he goes to vacation in Unova he's a bit rusty, coupled with Zekrom resetting Pikachu
(Very loose explaination ik but it can still fit)
I'd argue BW - XY Ash had a perfect transition, BWs end perfectly sets up Ash being disappointed at his performance and aiming to do much better and be motivated to win, directly leading to XY.
After his fierce battle with Alain in the Kalos league and all the shenanigans, and reaching so close to the league and having such a great battle he doesn't even feel sad he lost, Ash decides to take it chill and relaxes, going to Alola for a vacation now. Deciding to approach things in a new way he tackles this region differently, living there and interacting with its culture while still being a very competent trainer.
And the transition to SM - JN ash is perfect as well
For me OS - JN is the same Ash, in some series he was a bit boring for some people, in some more interesting or charming but overall it's still the same Ash