I'm almost done with the 4th island after about 25 hours of total gameplay and 98 totem stickers. It's the same game so far. They can swap character A for character B and add some random new reason for this or that but in terms of what you do as a player, it's virtually identical (not including some new/changed minor sidequests). If there's going to be any new story content before the post-game other than the occasional reminder that Necrozma is just the worst (and making me fight the same mon 3 or 4 times that I one-shot every time now), then it's going to have to be literally the very end.
Most trials are slightly different (though some are the exact same) but I think for the most part they got worse. That said the totem fights got better. A lot of the allies they call are just deliciously evil and succeed brilliantly at being a pain. I never lost to one, but I came close a few times.
Oh and Mantine surfing is really not fun, but it's also overall inoffensive. Annoyed at my prize for beating all the top scores. Still, it's the most effortless way to get BP at a decent rate. Though it's definitely slower than a decent run at the Battle Tree.
I agree pretty much with your thoughts on the game, I finished it a few days ago after receiving it early, and overall it's the same game with the end changed (story wise), and some extra Pokémon that weren't available in S&M sprinkled in.
That said I really enjoyed it, and do not regret buying the game at all. It has many QoL fixes, and I'm excited to play more of the post game
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