This is not as easy as it sounds. For that, I would have to organize a regular copying of my data between “ordinary” and “data” qubes. This means a lot of cumbersome scripts to write. Also whenever something changes in the “ordinary” qubes concerning the data structure, the scripts must be adjusted. Not even speaking about verification of such backup: How can I make sure that I didn’t miss something important?
Do you care about possible Spectre- and Meltdown-like CPU vulnerabilities? Do you use your Qubes OS normally while backing it up or do you Pause All VMs for Secure Operation?
(Also, some slower machines might be not very usable while backup is being made.)
I believe that this relatively simple feature is not sufficiently prioritized in the Qubes development. It affects most new users endangering their data due to unnecessarily complicated verification… It’s a very old Issue and I complained about it before.
In any case it also makes the backups more cumbersome…