Asking how Gen 8 Ash should be seems like just another way of saying how do you think Ash should be best written in-general. However if it were up to me, I'd just reboot his character in Kanto but no. Some factors of Gen 8 still count, so a reboot in Kanto is off the table since it's still sent in Galar. Honestly though, I'd prefer it in a similar way of that of Pokemon Adventures Red.
I'd really like Ash's battles to be shortened in the same style as Pokemon Adventures, a battle of brains where you make key decisions that feel like they all matter in some way, outsmarting the other until one trainer lands a shocking "checkmate" on the Pokemon with a single but effectively smart blow that surprises the audience even more because they understand how it happened even though they did not see it coming, not just some simple dodging and brute force like previous battles. In order for Ash to do this properly, he actually has to battle extremely powerful and smart opponents like the Elite Four or Giovanni, so for my idea of this series, Ash also has to grow into an extremely powerful trainer surpassing that of Ash-Greninja but no power-ups, no mega evolutions, no gimmicks. I want to see something like Infernape's Blaze Ability go against all odds against the Mega Evolutions, the opposing Ash-Greninja, the power-up gimmicks that Gen 8 has. A moment like that would amaze audiences and if it feels earned, will really show how far Ash has become as a Pokemon trainer.
Limitations and understanding those limitations are key to making a victory feel more satisfying. Also of course, I do want the battles to feel earned and make sense but if I want Ash to be like Red, he has to be a genius at Pokemon battles, he has to use unorthodox tactics like before, such as Buizel's Ice Aqua Jet, that actually make a lot of sense. Red's finishing blow to Green's (Rival) Pokemon in Pokemon Adventures was amazing because it was not only unorthodox but it made sense despite the odds being against him. I want to see something like that in the anime against an rival who uses tactics more accustomed of the game's rules.