I do think this series has a good shot of using reserves.I agree with the Master class battles. I really hope they're not 1 on 1, make them at least 3 on 3. And the final battle against Leon has to be a full battle.
About the reserves, for me particularly at least, that's what'll make or break this series. It wouldn't feel the same for me if Ash won the PWC without using his reserves, than if he did it using his reserves. With just JN pokemon, it would feel disconnected from Ash's journey as a whole and thus deeply unrewarding. Using both JN pokemon and reserves, it would feel like the anime as a whole comes full circle and it'd be a lot more satisfying.
I mean, I wouldn't be remotely shocked if it doesn't (or only extends to the Alola crew) but...usually Ash only captures regional pokemon for marketing reasons. But obviously that's out the door, Ash isn't there for Galar marketing. Might as well market some previous generations. I doubt Torkoal or something would make an appearance, but Kanto mons or Infernape/Torterra/Gible (if there's a DPP remake coming this generation) would be totally fair game.