Thanks. This seems to be the way I have to go ✏ Update Guide for a modified FP Open OS (with Gapps, Xposed and more)
(including installing TWRP Community Port in advance because of the encryption)
I will let you know if everything went fine.
Currently my phone is hanging when I try to boot into TWRP (but I’m still able to boot into the OS). I hope I will be able to somehow start or install the community port of a newer TWRP release using adb/fastboot.