Did you check the package details for the Radeon drivers on the Arch site to which driver (if any) support the card? If it’s like Nvidia they have a list of cards supported for each driver. I assume they do the same for Radeon.
I have to remember to get the legacy Nvidia drivers now for one of the graphics cards.
They probably use HDMI 1.4 (or older) and not 2.0, meaning the total data rate is not enough for 4K@60Hz, you’re pretty stuck if that’s the case, just hope they have a display port connection and use that instead. I highly doubt that a change in gpu will remedy it as the 400 series was able to do it fine over display port.
Don’t bother with AMDGPU-PRO, the open source driver is just as capable and more regularly updated.
I’m using a Vega 8 integrated graphics chip (same generation as the RX570) with the open source xf86-video-amdgpu DDX driver and amdgpu kernel driver and the performance is better than I get under Windows 10, they are fantastic drivers.
The only extra configuration I use is the TearFree option: