Never mind, fixed it myself. =D
Here is how I went about it:
First I flashed the boot.img from the FP5-TT3L-factory.zip found here:
Presuming you have the extracted image in the current directory within your shell the command is:
fastboot flash boot boot.img
If you, like me spaced out at 2 in the morning and somehow accidentally flashed the wrong boot.img from the newer OTA FP5-TT3G-factory.zip and magisk complains, that there is no Backup to restore, when trying to use “Uninstall Magisk” and “Restore Images” just follow this instruction:
and make sure to use the proper boot.img from your currently installed OS version.
Afterward, I backed everything up to my SD card and also an external device, since I wasn’t sure that reflashing vbmeta.img wouldn’t somehow initialize a factory reset and wipe /data.
Better safe than sorry.
Now comes the actual fix that restored my ability to upgrade via OTA:
Extract the vbmeta.img from the aforementioned zip of your currently installed version and place it in your working directory.
fastboot flash vbmeta vbmeta.img
Feel free to ask, if you have trouble as well. I am happy to help =)
Looks like I was too quick with my write up, the OTA process checks the Hashes of the boot.img thrice.
Once while downloading and applying the update. (This is fixed with “Uninstall Magisk” and “Restore Images”)
A second time while optimizing the system before reboot. (The previous fix still applies)
And third time when rebooting the device.
So if you flashed to inactive slot (After OTA) the OTA will still fail. =/
I now got the OTA to work with basically stock everything, except for the unlocked bootloader, which means loosing magisk for every OTA as of now and reflashing the patched image via fastboot every time.