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