This is what I do: I have an external drive with a LVM volume for each qube I want to backup + one for the Qubes backup tool (I still use it for some templates and qubes). I attach this disk to a qube in order to decrypt it. Then, I can attach any LVM volume to its corresponding qube, mount it and do the backup (I use borgbackup).
Of course, the qube decrypting the drive could have access to all the data … I’m not concerned about this right now, but maybe a better solution could be this multi-layer encryption tool called qcrypt: GitHub - 3hhh/qcrypt: multilayer encryption tool for Qubes OS
As for Salt, I don’t think there is a way to generate salt states from templates. It requires more time but will give you a better understanding of Qubes OS. Leo wrote a good introduction there: Qubes Salt Beginner's Guide