To your last point, it felt like they had to rush a lot of necessary plot beats to fit the movie in the time allotted while still also getting in all the "named characters" they had to. DK seems to have been the most ruined by this, but basically nobody but Mario and Luigi get the time needed to actually feel fleshed out.

That's why there was so much debate about Peach's line about clearing the course the first time. Because its both in character for her to brag about that while leaving out a lot of context or just being an awesome girlboss with no flaws, which is only an issue because her backstory is so rushed that we have no idea about how she got so "competent." If it was natural or did she actually work just like he did, just from a younger age.

Honestly, if they had just cut all of the Donkey Kong parts (and with it the kart segment) they probably would have had a lot more breathing room to fill with development to make Peach less generic, because that's what she was boringly generic. Those sequences were awesome, but they fill a lot of runtime that other characters needed to actually exist in.