This suggestion is mainly intended for templates (e.g., list of packages to install, steps taken to configure and customize programs), since the data is publicly available on the internet and can be redownloaded. However, most users also have irreplaceable data (e.g., photos, emails, documents) that can’t be recreated from just notes.
So, could one just back up AppVMs frequently, and templates perhaps infrequently but then again, perhaps not at all? (If your template creation is scripted or salted, then in theory you need never back them up, BUT you will want to back up your scripts and/or salt recipes, and you will want to do so THOROUGHLY (offsite backups, the works.)
By AppVMs I am talking about ones that the user gets onto to do things to his/her own data, not anything at the more “system” like level. Even though almost everything I have sits on a NAS in VeraCrypt containers (and gets backed up six ways from Sunday), there’s still a little bit of stuff on AppVMs I want backed up, but then, those tend to be the smallest VMs in terms of usage anyway. (At least they are if the data is minimal–the point being you won’t be backing up copy after copy of the same dang OS.)
Also, unfortunately, a lot of user settings live on those VMs, which is preferable to them living on templates IMHO unless you can script setting them.