Sadly I have to echo R_N , I've ran the entire game recently (took me a while lol) with a full 6 members, and the pokemon that I did not change from the start were all brutally overleveled all the way until Cynthia.How? We're you running through that "substantial chunk" with a team of less than 6 Pokémon?
Or are you referring to the part between Maylene and Fantina, which is caused by Wake and Maylene having the 3 Pokémon be the exact same levels as each other, and isn't really something that can be worked around without changing the levels of Wake's Pokémon and every other important fight afterwards. Cause that's just kind of the result of the level curve going completely flat for a bit and has nothing to do with the different Exp system.
In fact... the funny thing is that since i *DID* change some members near the end, they didnt manage to catch up, and let me tell you, fighting E4 in that game with non-overleveled pokemon was no joke and I actually only won on first try cause friendship bonus made my Gastrodon survive +4 garchomp's EQ and be able to fling 2 Ice beams instead of a single one.
However I still don't understand the complaint for EXP share, mainly cause the issue isnt exp-share per se but just the bad level curves of the games. Most modern JRPGs with "potential party members higher than current active party members" feature some sort of exp share in order to let you always use the members you want at a given time without need to level them separately, and not punish you for having leveled the "wrong" party members and suddently need the other ones who are 40 lvls below.
Plus, I'm always of the club of "If you want to play a challenging game, go play something else, because Pokemon games are not designed to be challenging, they're designed to be fun and kid friendly".
What however I *DO* wish they would remove or let you turn off is friendship bonuses. They just feel... wrong.
They're also ridicolously overpowered, mainly cause you get access to them while the opponent can't. As I said, winning against Cynthia just off a lucky survive from my Gastrodon didn't quite feel right nor satisfactory.
I can definitely see a kid be happy for it, "oh wow my Gastrodon liked me so much he endured the hit :) ", but... I just think that kind of bonuses shouldn't be obtained by default like it is in BDSP, and should at very least require you to actively do something, kinda like in gen 7 you had to use pelago if you wanted to get these bonuses.