I can say, I understood a limitation of amdgpu kernel driver has been not supporting VGA/analogue outputs in some (or all?) cases. I understood this is part of the reason it is not the default driver over radeon for some cards.
However, given many other other experiences and bugreports with amdgpu modesetting problems, and looking at this bug, I would recommend trying newer kernel in buster:- enable backports https://backports.debian.org/Instructions/ apt-get -t buster-backports install linux-image-amd64 linux-headers-amd64 This should get you a 5.10 series kernel which could well behave better for you (let us know!). --Simon p.s. the backports kernel is lagging behind a little, hopefully 5.10.0-0.bpo.6 and 5.10.0-0.bpo.7 will come out soon.
