Ok - I did it. I’ve taken the “upgrade” to Android 13 as a chance and did a complete fresh install using the factory images from here: Installing Fairphone OS Manually – Support
However, I’ve stucked into the “critical partions locked” - problem. Almost same like described here Stuck with locked critical and FPOS - Help / Fairphone 4 - Fairphone Community Forum, also that solution worked for me. (Thanks @ @hirnsushi)
Additonally, I found out that my “active slot” was set to “B”. So I reset it also to slot “A”.
Even I was fine with my backup strategy (Swift backup + some manual messenger backups), it tooks a couple of hours for me.
But now I have a rooted (Magisk Delta) FPOS 13 with all my needed apps incl. data/settings.
(Even some AdAway problems seems to be solved now, the main reason to root for me, the only thing I’m missing is a reliable vanced youtube replacement).
I’m looking forward to the next OTA and hopefully the build-in magisk mechanism (install to inactive slot) will work then 