[Quoth]

KFX is mono unless a colour device and tailored to the device.

The azw3 is now only on the K3 to PW1, all newer devices get KFX.
The old mobi (KF7) is only needed by K1, K2 and DX family. Mostly they can't get delivery now, though the International versions of DX (DXi and DXG) may still get delivery via EDGE version of 2G-GSM.
The azw3 & old mobi have colour images/covers if that's the source.

Amazon always ditched fonts if "too many" and resampled images if "too big" on all formats.

Also the source is often epub and only the azw3 is close or identical. The old mobi is terribly degraded (HTML3, no CSS & limited fonts) and KFX conversion is Amazon's concept of a conversion, so while full featured, it's less like the source than azw3.

I've switched to using a K3 (gen 3 / Keyboard) which got azw3/KF8 added in a FW update in 2017 instead of the PW3 (gen7?) which now gets the less good for conversion KFX.