XP-> Ame is or was gonna add Rejuv's Level cap system for e19
Fields-> Fields are what make the gym fights hard though, its personally nicer to beat the leader when its on their perfect advantage (Here's looking at you Charlotte and Amaria)
EV/Stats -> Yeeting set up moves = a lot of work to code removal/Cap limits of setup as some mons complete abilities are dependent on that [Moody, Hyper Cutter, Belly drum loses all usability along with anger point ], All the gym leaders mons to my understanding are Max EV IV trained anyways
Common Candies-> Point becomes invalid for e19 getting Rejuv cap changes if first point is still correct, plus ive used maybe 5 common candies to my life in my current 154h of playtime, plus you have SMW/Additional Options modpack to fix that cap issue anyways
Mining rocks-. are mostly whatever cause the big one heart scales i believe are in the pickup table, so why mine when i could just run around and get a buttload with 6 pickup mons fighting trash wilds?
Im only starting an intense mode playthrough for Rejuv but i had issues in normal mode reborn and rejuv normal mode getting past parts so im not there with difficulty
Id recommend looking at Pokemon Reborn Redux as it gives each gym leader much harder teams to fight against as a lot of the changes your asking for requires a lot of effort recoding the spaghetti code that is essentials, changing entire teams of leaders, recoding the fields system, recoding the evs/ivs, recoding stat setups cause changing that code affects Opponent too
Very very large amounts of essentials code changing
-Personal Opinions be this thread as a casual player who hates Ciel after being stuck on her for 3m