Hey
I built my first one too, exactly the same problem with 6700xt
Bios and boot logo green and on the right of the screen enlarged
But windows is fine
Annoyingly though it was on a 4k tv as you
I will wait for my 27 2k monitor to arrive and i will let you know .
Many have suggested its the screen not agreeing with the updates
Switched also to csm bios instead of uefi but same problem, even flashed the motherboard bios to new version
All same
Soon i will tell you whats up