This is repeated frequently, but I was never able to get my AMD 560 to work on Linux at 4k with MST (or turn down the fan speed). With Nvidia I installed the binary blob and 4k over MST “just worked”. I tried the amdgpu and Pro versions of the drivers.

Neither company is perfect with Linux support. As with most things, there are pros and cons to both sides. But for anyone else reading here, just because you pick AMD does not mean everything is magically trouble free on Linux.