I think that overall, whilst the game definitely has its difficult moments, it's not totally out of proportion outside of a few moments. One thing I have noticed is that Fog of War maps have become considerably more dangerous, with the AI sometimes coming out of nowhere and one-shotting some of my team members. I've also noticed that if you don't train some units early game, it doesn't take long before they become incredibly difficult to train without extreme meatshielding (this happened to me with Artur).
I'd say the easiest way to rebalance this hack, but keep it reasonably difficult would be to have a more paired back skills system (similar to Vision Quest) and balance around the fixed growths patch. One problem I always found with random growths in FE is that it always feels like the games have to be balanced with zero growths in mind, given you could get really unlucky in your level ups. With fixed growths it's easier to fine tune levels given you have a reasonable indicator of where a player should be with their units for a given point in time. I also feel that it could be good to make Easy mode like FE8 normal mode, Normal like FE8 Hard mode and Hard like FE7 Hector Hard Mode, to allow for a full spectrum of people to play the game (I know you have partially done this but I'm not sure how much you have fine-tuned it for different difficulties).
Overall though, the things you have added to the game are so enjoyable that the difficulty is not something which gets to me too much. I'd honestly love if someone could do the same you have done to FE6 and FE7.