That does remind me - PLA made a few changes to the damage formula, and among them was the fact that level scales stats a lot less, meaning lower leveled mons can still go up against overlevved mons pretty decently. Now, this could just be a Legends thing as that game basically rewrote how battles work, but I think that with an open world title like this, it could apply here as well. You as a developper won't have to worry about a player sequence breaking and getting a lvl 40 before the first gym if you make it so being lvl 40 doesn't completely trivialize battles against lvl 20s.
(Plus personally I think its just a generally good change to the damage formula. People have been complaining for ages about how you can just overlevel a mon to get through the game or how the exp share leaves you overlevveled. Reworking *how much* level affects damage is a great way to work around this)
In which case, another way that they could dogym progress is through stats. Every totem in Gen 7 was EV trained, so I can maybe see them doing something like making the first gym you face have less optimal stats and add more EVs/IVs as you gain more badges. Most players don't really bother controlling how their mons gain EVs, much less getting good IVs for an in-game team, so this could be a fun way to give the gyms an advantage over the player. I'd usually not be so optimistic but BDSP delivered us a lot of competitive stat spreads, so I can kinda see SV doing the same for late game trainers. Would be pretty funny for Little Timmy to try being a smartass by leaving the earliest gym for last, only to get destroyed by a fully EVd lvl 20 Sudowoodo