There has been a development
I have discovered that turning the screen off and then back on again with
xset dpms force off
xset dpms force on
fixes the issue, but this has to bee done every time it starts flickering, so after most reboots, and after closing and opening the laptop; so this is a relatively good workaround, at least no more rebooting sagas are needed to get it working.
Also, I just updated to the latest stable update ([Stable Update] 2020-11-04 - Kernels, Nvidia 455.38, LibreOffice, Plasma5, Frameworks, Apps, Gnome 3.38, Deepin), but that did not resolve it unfortunately.