Finally succeeded with 0.86-installer (appimage).

Flashing phone to required android build using fastboot and flash-all.sh https://dl.google.com/dl/android/aosp/sargo-pq3b.190801.002-factory-56817dc2.zip
worked without problems.

Experience with snap ubports-installer 0.89 and 0.91 was frustrating.
In many many tries phone was not recognized and connection could not be established. In those few cases when connection appeared to be established installer never actually progressed to flash the phone though announcing that.

Using Appimage ubports-installer 0.86 https://github.com/ubports/ubports-installer/releases eventually did it, at first try.

Difference: First job it reported was to change adb-udev-rules and then finished within 15 minutes.

btw: Google-Screen at boot is funny!