There are really 2 problems here, but at this point, I just want a clean re-install of the Ubuntu partition.
I used the same laptop with Win10 and Ubuntu 16.04 for several years with no issue. Then I step-wise updated through the LTS versions to arrive at 20.04 a while ago. Initially, it worked fine, but at some point, I probably screwed something up. The result is that the moment I log in into Ubuntu, the screen freezes up completely (no cursor movement, no reaction to keyboard input, no way to get the terminal up).
Every once in a while it is still functional, and I went through all tutorials on how to fix graphics card issues etc. as this seems to be the most likely culprit. Nothing helped. I managed to save all my Ubuntu data. Now all I want is to safely do a clean re-install of the Ubuntu partition without messing up Windows. How do I do that?
I see various partitions on Windows in Disk Management (including the "primary partition", which seems large enough to be Ubuntu) but am unsure how to go from there.