Got a 6800 today, same problem. So the fun begins 🙂
Will report if I find anything.
Edit: Haven't had luck until now. Tried manually updating linux-firmware, kernel 5.10.2, mesa 20.2.6. But after updating linux-firmware my system crashes instantly when I start X Server. Maybe newer llvm version is needed as well or xorg-server. But I don't know.
Edit2: @littlej0e I tested out another OS (Ubuntu 20.10) to rule out that it's a hardware problem. If you don't want to play on Windows that would be another option. It's relatively easy in Ubuntu to get it working.