I think an aircraft in the class of the A400M exactly as you mentioned could be indeed quite suitable however it would require a similar powerful engine and it seems China still have issues with less powerful ones.
it just a plane that sits between the Y-9, and Y-20 can't quite fit itself in the doctrine.
I don't see the PLA need a plane that can carry just 1 of the 8 wheeled chassis for the medium brigades for example .
Since China has lower global rapid deployment need ,there isn't a requirement for mobilizing a "striker BCT" half way around the world to a spot that has beardly a runway, not now anyways.
The same money would be better spend buying more Y-20.
The top priority these days for the PLA is transitioning as fast as possible to an threat that will prevent USA intervention with in 1st island chain , that means more tankers , air control asset , ground attack asset, air lunched hypersonic missiles for the PLAAF , gaining the ability to go out and take the enemy air force out, combined Air supremacy & SEAD.
With these goals in mind a medium sized transport just isn't high on the "Wishlist"