I finally updated the firmware on my Mac Pro 5,1

[MrNoBody]

AllGold wrote:
...
(The RX560 will only work with 10.13.6 or newer. If I want to go back to
an OS before High Sierra for whatever reason, I will have to swap out the
video card.)

It's not the RX560, it's the NVMe. Sierra doesn't recognize them.
I have an RX570-8G which works fine with Sierra. However, if I boot to
Sierra, I cannot restart to Mojave. I have to restart to HS then restart to Mojave.
(Mojave on NVME, Sierra & HS are on a PCIe SSD controller)

On Sierra, Sys Info show this for the RX570:
System Version: macOS 10.12.6 (16G2136)
Chipset Model: AMD R9 xxx

[AllGold]

Ah yes, MrNoBody, you're right. Drivers for those video cards first appeared in Sierra 10.12.6, not High Sierra 10.13.6.