The gap of Futsuu and Muzukashii is too large.
As we have RC here:
"There cannot be any drastically large difficulty gaps between any two difficulties."
Currently, your Futsuu having tons of 1/1 patterns but quite less 1/2 patterns.
However, the Muzu having a lot of complex patterns which are complicated with 1/2 and 1/4 triplets.
Pretty obviously that the spread is not smooth as the density of Muzu looks like a general Oni and Futsuu looks like a Kantan having some 1/2 patterns.
I suggest you to do both things:
I. Buff the Futsuu, through adding some more 1/2 patterns, like how you put triplets in Muzu.
II. Nerf the Muzu, especially the density, because it doesn't have too much gap between Oni.
roses are red, violets are blue, but i don't think i'd really know if i was a 500 pound elephant who is colourblind and illiterate.