I had this same problem, with an RX570. What finally worked for me was to use spaceinvader one's user script to dump the vbios. After that, things finally worked and I could install the most recent AMD Adrenalin drivers without getting a black screen. I did have to restart the VM a few times at one point in the process.
Some other details that may be relevant to my answer working for you:
- I installed windows 10 pro directly on an NVME drive, then created the VM to use that drive
- I had to create and delete several VM Templates with all the different things that I tried - switching between the GPU passthrough and VNC did something funny at one point
- The last thing I did was to boot in to windows safe mode, use DDU, then I made the template with the vbios and was able to install drivers in the VM
- When the dumped vbios was linked in the template, I had already installed the vrtio drivers. I don't know if that order matters
- Using the vbios from TechPowerUp didn't work for me (i didn't check if there was a version mismatch)