the community branch offers those versions for the FP3. I read of the Backup option happens to be hidden in some devices (depending on upgrade path?).
You should give it a try, even if it’s still the old Seedvault version in A13 (I’m actually not sure what it ships, it can tell you in the upper right settings what version it has). At the very least it will transfer all apks to the FP6. How much settings are transfered you’ll need to find out.