>>48032629USUM are my personal favorite main series Pokemon Games
BW2 challenge mode are the only other ones I'll MAYBE consider playing, or maybe XY
but even with hardcore nuzlocke rules, they're easy, while USUM (still not HARD if you're replaying) is the most challenging
I do like doing similar strength randomizers (only wild pokemon, trainers are the same) when replaying a main series game nowadays to allow for more unique and diverse teams.
The only flaw in terms of replayability is the dialogue/cutscenes, and "difficulty"
Both of these issues are easily fixed, "too hard" just get good, they are not that hard at all
And as for the speed issue, if you play emu you can just- speed up...
If you're on cart, honestly just suck it up. I'd rather spam a while watching youtube or some bullshit then play any shitty ass gen 1-4 game.