I totally agree and share your concerns when it comes to MariaDB data safety.
That’s why I included a MariaDB SQL backup solution with self-hosted Murena NC23 release, based on [HOWTO] Properly backup self-hosted /e/ cloud databases.
It’s not activated by default, owner’s choice!
Backups are simple dumps to make restore easy, they are plenty of guides around (for example, here or here).
I made it modular, so it’s easy to add a supplemental “customer” database, just by duplicating one of the provided script and SystemD configuration files.
Also, it shouldn’t take much work to adapt for another MariaDB (not Murena Cloud) instance.
Please ask (privately if you prefer) if you need help for these 
As an alternative, any snapshot method for a DB engine should be taken with DB engine off (ie. stopping the Docker container). My choice for self-hosted Murena Cloud goes to shut down the entire VM, to ensure consistency.
Sadly, I couldn’t find any easy solution about downtimes when upgrading the MariaDB engine 