Since I had an issue with the root software (I had supersu installed instead of magisksu) I just reinstalled everything.
Before following the usual process though, I did a full wipe, including a "system wipe" (which I didn't do on the first try), with TWRP.
Then I flashed the firmware, flashed the rom, and the gapps.
No more FC at the first boot. Looks better !
I think doing a "system wipe" is important (the default TWRP "factory reset" isn't enough).
Yep, always clean flash is recommended.
Clean flash = wipe system, data, cache/dalvik