The issue and solution: https://ubuntuforums.org/printthread.php?t=2396045&pp=10&page=1
That only took two days! :)
Many thanks to @ciaby for helping me :)
Now: do I try installing Pop!_OS again (initially it didn’t even boot the live disk on stock 16.04 on the XPS) or leave well enough alone for now? :)
This is my personal Mastodon.