To close the loop: I succeeded to install LOS+microG on another FP3+ with images from 20221201 (recovery) and 20221204 (zip). 
(score is now: 2 success - 1 brick).
Here are some of the things I learned / did not learn on the way:
Not working:
-
fastboot boot <img>withoutflashconsistently triggers a normal boot, so it doesn’t help on the FP3+. Tried both the plain LOS and LOS+microG variants of the recovery image.
Working (based on https://www.reddit.com/r/LineageOS/comments/n0k6sn/comment/h8ygk41/?utm_source=share&utm_medium=web2x&context=3):
- detect active slot (when device is in the bootloader):
fastboot getvar slot-active:afastboot getvar slot-active:b
- flash depending on the output from above, e.g.:
fastboot flash boot_b <recovery_img>
No answer found:
- Some posts suggested
fastboot flash:raw boot_b <img>(e.g. Unable to boot into recovery - #17 by Nicoolas), but I found no suitable description of the differenceflashvs.flash:raw. - I’m also not sure if flashing to a specific slot is needed, rather than just saying
boot. - What exactly caused the procedure to fail on the previous device?
- The recovery image for LOS+microG still has no sha256 to verify against.
- One guide suggested the command
fastboot boot recovery <img>but I couldn’t find any confirmation that this syntax is even accepted.
Wiki update?
- I guess ✏ FP3/FP3+: Complete guide for installing LineageOS should be updated (that’s where I read about the not-working
fastboot bootvariant).- I’m not confident to directly edit the wiki
- its meta discussion is closed, so I can’t reply there.
- what would be the normal process in this case?