The MM already provides monsters up to 10 levels higher than PCs can ever reach in 5E... that's "epic" built right into the core monster math, right there. This all seems very... Krusty, which is enough reason to give one pause on its own. There are interesting concepts here, don't get me wrong, I'm just not sure what the point is unless PCs either get bigger Boons or more levels, neither of which necessarily seem like good ideas. It's hard enough getting a group to 20, never mind whatever hoops the DM has to contrive to justify Epic Boons. Putting currently-reachable entities like Orcus into these "beyond the pale" mid-30 CR range is "niche" to say the least, but "bloated" feels more like it.
I think if I had my druthers we'd acknowledge that PCs levels 15-20 would be classified as "epic", while monsters CR 21-25 would be classified as "immortal-tier" (c.f. ancient dragons, kraken, titan), CR 26-30 would be "lesser deity"-tier monsters (c.f. Tarrasque, Tiamat, and Orcus), while CR 31+ would be "greater deity"-tier monsters and possess party-busting fiat-like powers rather than arbitrarily high DCs, ACs, etc. I mean, at a certain point PCs have no hope against these things, why spend any time doing table math to prove it?