(From nvidia-detect)
$ nvidia-detect 10de:0391
Checking driver support for PCI ID [10de:0391]
Your card is only supported by the 304 legacy drivers series, which is only available up to stretch.
In other words; downgrade Buster to Stretch if you want to use the nvidia-graphics-drivers-legacy-304xx packages
Maybe you can install the 304 driver, but you must downgrade the Xserver to version 1.19
And this is what was installed, as I've now removed it
with apt-get purge nvidia*
nvidia-legacy-390xx-alternative (390.132-3~deb10u1)
Thanks.