I managed to take some time and verify my findings.
It looks like I made mistake on first installation because today I managed to flash Focal directly over FPOS. So right now my flashing procedure was like this:
Download and Fairphone OS based on Android 10 Flash Fairphone OS without locking bootloader Boot into Fairphone recovery and erase user data as requested Reboot to Fairphone OS welcome screen Reboot to Fastboot mode Use Ubports installer and let it do its job with Focal/stable branch