I think implementing core game mechanics too late in the game is an rpg no no, mega evolutions being implemented in the last patch, hell even the last couple patches is bad because while it does preserve difficulty.
These fights aren’t really difficult past narcissa since you have a wide variety of Pokémon to choose from and essentially infinite rare candies with breeders. Mega evolution being added before narcissa would be op, but all of the gyms after that are largely irrelevant do to the breeders making experience a moot mechanic and you don’t really need to worry about making a well balanced team that you dedicate your exp to; When you can just catch the counter pick and train it in about an hour.
A lot of the fan games that have mega evolution, have this notion that it should be added late into the game but by the time you get it, the player largely has little to use it on. They should honestly take a page from X and Y and give it to us early instead of Sun and Moon when there is little to do with it. It’s worse here because at least those games have competitive scenes but I doubt Rejuvenation is getting a meta and with this game’s dialogue heavy patches. The gym leaders and notable battles should be challenging and incorporate all aspects of the game by this point since trainer battles don’t actually matter anymore. (V10 only had like 4 notable encounters, Aelita rift, Souta, Vivian and, Kenneth some of which had mega evolution)
Tl;dr I would say adding Z moves and mega evolution while using even more powerful field effects towards the end of the game would allow for more diversity and allow specific encounters to shine. Instead of all gyms being rendered to, go catch that Pokémon and run through, we should be given all aspects of game play, by at least crawli and be allowed to fend for ourselves using the tools we’ve been given.
But that’s all my opinion.
(Edit- if some mega want to be held for late game such as mega Salamance or mega metagross because of how op they are, that’s fine. But some such as Mega Audino don’t really need to be held back that long.)