For that card, you probably have to set the radeon.si_support=0 amdgpu.si_support=1 kernel options to allow amdgpu to work. I don't have a TrueNAS system laying around so I don't know what the idiomatic way to change them is.

Using amdgpu on that card has been considered experimental ever since it was added like 6 years ago, and nobody has invested any real efforts to stabilize it. It's entirely possible that amdgpu on that card is simply never gonna work. But yeah I think the radeon driver isn't really fully functional anymore either, so I guess it's worth a shot...