You’ve raised some fair points especially on boss variety and world depth. A quick question, do you think these design choices were aimed at a younger audience or just development constraints?
I don't think its either, even though Mario games are always light-hearted.
I don't actually know how the Super Mario dev teams' mindsets are, nor how they work, but I kinda feel its not from dev constraints. What I think is that with all their time spent on making new environments, enemies, and levels, they kinda not bothered with sparing time for making another new array of bosses in Wonder, intentional or not. All we got were Bowser Jr. and Bowser as the game's only bosses. That is what I find EXTREMELY disappointing in a new Mario game.
And, no, the Koopalings don't need to be constantly reused again. I mean, they're alright as bosses of their own, but new villainous talent should be made a bit more often.
I've seen more creative minds that employed lots more variety than that, and especially with loads more game storage space we already got in many outlets that doesn't warrant less bosses in any way.
What I always expect from any new installment are not just new enemies, but new bosses; not reskins, resizes, or recolors, just new, original bosses. And Wonder clearly doesn't have any. Period. Not even a single chosen enemy turned into one by any resource needed, even by a Wonder Flower.
Some past Mario games had them step up a bit in bringing new bosses, but that's been kinda lacking lately, and that can be very harmful for later, new games and other devs trying to seek new inspiration sources for theirs.