I figured out that the graphics bugs depend on the desktop environment used. Tried out the listed ones here and most of the desktops do not show the graphic artifacts. Only Zorin Desktop and Gnome with their "fancier" graphic elements do.
Ah, the 6.8 kernel did not change anything, so i stayed with the 7.x one.
Telling me now that I "literally using unsupported hardware" is easy for a linux novice. Yes, the hardware is old, but should I only have known that its not supported just based on the age? Where is the step from unsupported to supported then? GeForce-100-Series? GeForce-400-Series? "Old" is a flexible term. In fact the hardware works well, with the right choice of desktop environment.
At the end I stay at the point that it would have helped me a lot, when on the hardware requirements page a sentence about Nvidia graphics support could have been found with a concrete series. Please consider to add it. You can also say something like "older cards may also work".