I run LG Ultrafine 4K + Packed Pixels 9.7" 2K (2048x1536) on Mini i7/32GB RAM.
FCPX runs pretty poorly. But not because of the screen, because the GPU in the Mini is rubbish. If you want to do decent stuff in FCPX, think about running an eGPU.
It ran just as poorly on a single 1080p screen.
for everything else its smooth and well.