Okay, all done with Hard mode! Solid conclusion to a solid game. Loved the final fights; I found DEUS much harder than the dreaded W.T., but I think that's because my team was ill-suited to it in particular.
I think I now have a clearer idea about the difficulty weirdness. The crux of it is, as I alluded to earlier, that the difficulty of the regular trainers is too close to the difficulty of the bosses.
Normally when I'm playing a game like this, I want the challenge level to have peaks and valleys; after a boss, we return to (slightly harder) normal goons and wind back up to the next peak. The regular goons in Empyrean.Hard are so chonky that there aren't really any highs or lows from a difficulty standpoint. I never really struggled with any of them, but each one was tiring, and there were a *lot* of them. So, they didn't really add difficulty to my run, they just sort of protracted it. Also, the rewards for beating a trainer get smaller and smaller the farther you go, and eventually they disappear entirely; I really don't need more money, guarded items have to be VERY attractive to be worth the time, and the experience yield is so low at this point that it'd be faster to use the daycare. This resulted in my doing something that I almost never do in any game, which is skipping fights. That said, as soon as I got to a boss I was having a blast again; I had a ton of fun with the Elite Four.
I am of the opinion that the solution is to greatly differentiate the bosses by highlighting their difficulty. I would try to accomplish this by leaning into the boss and miniboss trainers more, and letting the regular trainers be more cannon fodder-y. Maybe it's just that I'm coming from Reborn et al, but I'm excited to climb walls like Crow and Emmanuil. I don't mind spending a few tries on a brutal gym leader, or even grinding up a new dude; I signed up for Hard mode. When I get to the boss room, I want to see something that makes me go "HOLY HELL", because that makes it satisfying to overcome. All of the final bosses, both individually and as a group, were satisfying to overcome. Fisherman Jeremy is not satisfying to overcome, even if his Swampert is technically stronger than my ace, and even if there are 27 of him between me and the next plot point.
Empyrean has an interesting quirk when it comes to difficulty balancing, which is that the GP, shiny factor, and cards make the stats really, really high-variance. The difference between a 5*/plat-card/shiny dude and a normal one is, if my sources and math are correct, roughly a 254% increase to everything, which is about the difference between a level 40 Glalie and a level 100 Arceus. There's a real concern about players being able to basically ramrod their way through things if they've done the legwork (which is sort of what I did; I barely touched anything but my Absol until levels were well into the 100s, but I also caught an unholy number of Absol at literally my earliest opportunity). Nevertheless I'm gonna argue that, at least for normal trainer fights, it's okay to let that happen. It gives the player a chance to feel epic before the next boss puts them back in their place.
So, my suggestions for Hard Mode would be:
*Pull the GP rating of the random trainers into the 0~3 range, inversely scaling with how many dudes they have (and reduce the average dudes per trainer in general; it's almost always three right now)
*Randomize their cards between tier 0~2, biasing heavily toward 1
*Drop their levels to be below the current soft cap
*Take away most of mega stones, so that the player cares when a trainer has a mega
*Take all of the GP, cards, and items off of the "training" pokemon
*Let the bosses be the ones with a high-GP ace or a platinum card. Let that be a special moment. I get the feeling that this might be how it is on Normal? I may go check when I'm less burned out.
Additionally, I would consider lowering the power of GP and cards, just in general, so they aren't quite so polarizing. Perfectionist that I am, I'm pretty much souped up with shiny stuff, but I can see a less stubborn player struggling mightily. I think GP becoming 1.05/1.1/1.15/1.2/1.25 and cards becoming 1.1/1.2/1.3 is actually perfectly reasonable, and that would still yield almost an 80% boost over a normal pokemon -- still massive, but no longer insurmountable (I do like seeing those giant numbers, though -- Absol just passed 1600 attack ^~^). Then it might not feel quite so mandatory to have every trainer be 5*/Shiny-Card on every pokemon.
Anyway, there's my five-paragraph essay. Thank you again for putting this game together; Empyrean is a beautiful piece of artwork, and I very much look forward to seeing what you come up with next.