I run production and test VM's on Synology NAS. For backup & disaster recovery, I have the option there to export the shutdown VM and import it if necessary. For intermediate steps before updates I create snapshots. I use both of these on a regular basis.
However, when reading the contents of the linked Backup & Disaster Recovery website, I have doubts that such a backup is suitable for simply restoring after a major upgrade. What I read there rather leaves the impression to create a backup, which can be easily restored in a version clean installation, so I could do that with the above mentioned exports and snapshots. But our topic is something different because of major version jumps.
For example, if I have a directory in an installation that contains the complete setting and user data and only that, it is very easy to bridge a major version jump, because I copy the directory into the new installation and it runs as before. If the MariaDB would be the database that controls and maps everything, it would be a further simplification, because then the content could be transferred easily via a transfer script.
Now then, these are thought experiments for a simple handling of such upgrades. From the current point of view, I would not consider any of the given ways as applicable and safe. Maybe the easiest way could be to transfer user data between two running installations via a command sequence.
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