Thank you, I had no idea, I will try this with LineageOS 16 instead and leave 17.1 to the more brave of heart! (My original software version is, according to the 'getvar all' thing, 3.31.401.1, so it's the Pie realease from May 2019 as I undertand it.)
I just need a custom ROM that is easy to install, reliable and safe - I don't want to experiment any longer after spending 4 full days on this!
I also wonder how safe it is to use a custom ROM at all! Anything I should know from you or read about, on this issue?
For example, how safe will my passwords and password manager be? Can I re-encrypt my device? Should I re-lock the bootloader, unroot the phone? Or should I just revert to stock ROM if I'm not an Android expert, and if I'm afraid of Russian hackers and the CIA??