Another theory I don’t know if any one has said before
What if the miraculous holders are vessels for the pure power that the kwamis hold.
Plague was massively destructive without his holder, even when he tries to be calm. That’s why the miraculous jewellery was created.
Like a harness. But the holders have to be humans, so releasing all that power onto one human would be deadly (linking back to my mayura theory). That’s why the miraculous run out. It’s the kwamis way of taking all the exhaustion from their holders, and the food that’s given to them is actually there to replenish the holders energy via the kwamis.
Hawk moth is an adult so can naturally take a much harder beating than the teenagers. His kwami (I think dont shoot me) hasn’t been seen to feed on anything. This could be because nooroo is unwilling to take that exhaustion so let’s it flow to Gabriel (explaining in another way why he stays away from everyone). I imagine that Fu will be far more powerful than Nino but not on hawkmoth level because he’s old.