Thanks to a helpful soul in another thread.
A lot of people are having issues with new cards when installing 21.8.1 (or 21.8.2) drivers. In my case, when during installation the video output would usually reconnect a few times (the 'flicker during installation' thing that is normal) while installing these latest drivers the video output would stop working completely. For me, using DDU to wipe the drivers, restarting, then restarting again would solve the issue for a single boot cycle. Madness. Anyway, I've got a better fix now. Hopefully AMD can figure out what's causing this issue so no-one else has to jump through hoops but for now:
Boot into safe mode (restart three times during windows boot)
Go to power & sleep settings
go to additional power settings (on the right)
go to 'change what power buttons do' (on the left)
click 'change settings that are currently unavailable'
turn off fast boot
If I had known this was even an option on my system I would have turned it off as soon as I got any kind of boot issue. Thanks Microsoft.
Hopefully this saves some people from going through a lengthy and pointless RMA process.
It would be great if AMD could suggest that people do this too, maybe a splash screen when updating drivers?
