Extra storage purchased in iCloud looks like the best way for your Mom but be aware that there's no guarantee that, in the event of a disaster, the backup will work properly.
It would seem the bulk of the backup would be for photos. You can set up a second cloud service to backup photos and increase the chance of recovery of those.
Most everything else is settings and apps and those are pretty easy to get back in case of iCloud backup failing. One just needs to keep good records of login credentials for every app or service one uses.