I confirm that the RX580 does not have framebuffer/KMS support with the ATI
driver.
There is KMS support with the AMDGPU driver, sadly it requires some proprietary
blob :(

During the Guix days you mentioned that you went for KMSCON because of encoding
issues if I recall correctly.  So what about this: on startup, test if KMS is
supported.  If not, fall back to the default TTY, and too bad for the limited
encoding :p  Would that work?

-- 
Pierre Neidhardt
https://ambrevar.xyz/