Kubuntu 20.04 keeps perform black screen with NVIDIA
Give nvidia-460 a try. See if that works for you.
Also check that you have linux-headers packages installed. They don't install by default and nvidia drivers don't build without them. Yet they are not listed as a dependency.
Execute this command:
sudo apt install linux-headers-5.8.0-50 linux-headers-5.8.0-50-generic
Either it will say that it had nothing to do, or it will install these all-important but always missed packages.
Then try installing nvidia-460 driver by any convenient means.