On the parallel worlds concept, 5 doesn't really bring it up beyond the meta tie-in for stuff like the Entralink or Old-Save bonuses, it's mainly 6 and 7 where it's raised (and in 6 it's technically conjecture that never gets any chance to be verified because of Zinnia). 7 is the only game where Other worlds are a significant concept in the plot, though 8 still has the Wormholes implicitly because of the Ultra Beasts appearing in the Max Lair. USUM was less the "last" opportunity for them to do Rainbow Rocket as it was written, rather it was the only time in the first place where it would make sense to attempt.It occurred to me that USUM was sort of the last opportunity for Rainbow Rocket's concept to have worked. Gen 7 is when the series began to transition away from supervillain plots for its evil Teams with Team Skull, though Aether Foundation carried itself as a second Team (I always figured that Aether not being directly slotted in was because it was already shown off in the main plot). Teams Yell and Star are beneath the likes of Galactic and Plasma. Trying to skip the Teams of new gens in favour of the person driving the plot of the legendaries still feels like it would be clunky: why would a Rose that successfully contained Eternatus be doing outright villainy? Gen 8/9 also have much less emphasis on parallel worlds as a part of the setting than gens 5-7, so 'antagonists from timelines where they won' would more jarring to suddenly introduce.
As far as the "person driving the plot" issue, that really only applies to Rose in Gen 8 (and this is assuming they were to use him and not someone like Swordward and Shielbert with either the cover Legendaries or Eternatus due to their Dynamax shenanigans). The Gen 9 Professor would probably fit right in with the other Team Bosses, given their blatant disregard for people or a facet of the Ecosystem isn't very far removed from Magma/Aqua or Ghetsis/SM Lusamine (which I'd expect to be the depiction they'd go with hypothetically given they used Vanilla Maxie/Archie AND that Lusamine being more overtly a villain in Pokemon depiction). Pokemon's not exactly moving away from plots where the ultimate antagonist can be given 1 or 2 faces, Rose was just a bizarre case compared to the outright criminals in previous stories (and this assuming they don't finagle a reason anyway like The Darkest Day ruining Galar to drive Rose out of his world).