Excuse my ignorance, I'm not sure where to run that zfs command. I sshed into root@truenas.local and I can't find any files or folders named freenas-bootNormally, if you go through multiple updates/upgrades, then you will get more selection amongst your list of Boot environment.
However, seeing what you have, Default would have been a good ooption, but I douobt it contains much data as it doesn't use a lot of space. You can try the "Default" and see if you can reboot and have the drives already imported.
Otherwise, I would try checking if you have any snapshot that could exist on your boot drive. (I might have been wrong about the creation of snapshots during update/upgrade.
However the Boot environment seem to be stored under "freenas-boot/ROOT" folder structure.
If you have snapshots, which you can list with:
Then you might be able to clone the most recent one to look for the keys and such.