Until I see it in the anime, I won't believe Satoshi will catch the legend. I don't remember him ever interested in catching legends (He cared only for friendship with them).
I’m not sure why you keep trying to make this argument that Ash’s has to express interest in catching a legendary for him to get one.
Look no matter how you try to argue it Ash has a significant amount of Pokémon that he didn’t initially express interest in but he still caught any way.
For instance he never really expressed interest in owning a Dragonite but that didn’t prevent him from catching one. Also he has expressed interest in catching legendary Pokémon before as he expressed interest in catching both Moltres and Zeraora.
So no, Ash not specifically expressing an interest in getting a specific legendary Pokémon doesn’t mean that can’t get a legendary Pokémon.
Especially considering that if Ash could only catch Pokémon he was specifically interested in catching then he should be able to have a significant amount of the Pokémon he does now, as didn’t originally expressed interest in owning them.
What I meant was that since Pikachu got his G-max form, there is probably no chance for Satoshi to get the Mega Stone. For some reason, there hasn't been a single person with both Dynamax Band and Mega Stone.
Not every major battle that Ash participates in will be Galar, and we haven’t seen any evidence that Dynmax/ Gigantamax can be used outside of Galar.
So no, just because Ash’s Pikachu can Gigantamax doesn’t prevent him from using Mega Evolution especially when he has a Pokémon on his team that has a Mega Evolution and has been inspired by it’s role model being Korrina’s Lucario who is able to Mega Evolve.
So what if we have yet to see a trainer have both a Dynamax Band and Mega Stone? Ash had a Ultra Beast, a mythical Pokémon, two starters, a special form of Lycanroc, and 3 signature Z-Crystals along with 5 or 6 normal Z-crystals which more then any other character that used z-crystals in Sun and Moon.
So no, there isn’t really any problem with Ash using both Dynamax/Gigantamax, Mega Evolutions, and Z-moves.
Especially when it’s pretty clear look at his team and at the anime that the anime isn’t solely focused on promoting gen 8 but is instead focus on promoting all the generations.
If anything there is a pretty good chance that Ash will get a Mega Stone and use Mega Evolution during this series.