Tbh rainbow team rocket in the anime wouldn't have worked, unless it's kind of an endgame series. Imo it really didn't have a place in the anime narrative that SM was going with
Not the entire thing, they were setting up something with Giovanni and Necrozma, yet that kind of got dropped when it did. That popping up at the league and making trouble could have been close enough.
USUM also ran into the issue of feeling a rushed product. Just see how over the place Lusamine's character was there. They reused stuff from SM that end up clashing with how they wanted to write her for this new interation.
SM was the rushed product. Our characters didn't have closure, a LOT of things were left unresolved. The Ultra Beasts were completely forgotten about in the main story and became random wild battles. We didn't have that 8th trial/ gym. The pokemon variety stank and the trials in comparison to SM were VERY underdone, where most of the bosses were either jokingly easy (Vikavolt having all physical moves) or has fake difficulty written all over it. (Wishiwashi)
There wasn't anything to do beyond the main campaign and we were railroad through it to the point we didn't have jack of agency. (Why did we have to go rescue the two people that caused Alola all sorts of problems?)
Also, Lusamine was being written as an anti villain in Ultra. (Get the idea that she was being an anti hero out of your head, there IS a difference between an anti hero and an anti villain.)
An egotistical loon that got screwed priorities in love and wanting to protect others, that took it upon herself to go after the looming threat of Necrozma in a style that only Leeroy Jenkins would understand. She turned the looming threat into an active one and caused ALL the problems we had to deal with in the main campaign. She spends the last stretches of the campaign and the post game making amends after everything blew up in her face. It just wasn't required in the main narrative for you to go around at the post game and see the process and the REAL problem with the Aether family in this story is that Ultra's story isn't about them. They are still important though, but the focus isn't on them, it's on you this time.
Since Masters is using Ultra's outcome for the Aether Family's follow up, that ought to be a good place to connect the dots.