UPDATE
First I have to enable developer mode and enable usb debugging again, no problem.
First real problem comes when wanting to flash TWRP. Guides (e.g. here on /e/ community) say to
fastboot flash recovery twrp.img, but that gives error
FAILED (remote: partition table doesn’t exist).
The reason, as I found out via searching, is that Fairphone 3 supports A/B partitioning, which led me to this explanation and guide - there is no separate recovery partition on Fairphone 3.
I don’t think I’ll have more time for this today, so new update (and hopefully success) tomorrow.
(I may also add that, as I do this from MX Linux, I have to do sudo on all fastboot commands, e.g. sudo fastboot devices. If not, I get an error: no permissions (verify udev rules) )