I can report I've finally managed to boot into 11.4 solely with my Gigabyte RX6800 card on the B550 Vision D. I had used iMacPro1,1 with agdpmod=pikera to boot in and it had to be on the 2nd HDMI slot. One very strange thing I discovered is that the card takes a long time to be recognised before I see the macOS login screen - a delay of perhaps 3 minutes or longer and that it doesn't wake up the display immediately if the screensaver has been set. Also the framerate performance seemed a lot slower than working in combination with the RX550.