How are you running Docker on your Ubuntu host? I had weird errors happening and realized that while I had installed Docker manually using the convenience script, at some point the Snap version got installed and that was a mess.
Switched to Debian, docker installed with convenience script, Nextcloud AIO has worked like a top since. All running as a VM on Proxmox as well, the host runs dozens of containers including a mailcow instance and several other stack beside the AIO, zero issues. I snapshot the VM before I do anything major like update NC so I can rollback easily. I might lose a few emails if I had to roll back, but never had to. Besides, everything is backed up via PBS hourly, so if I had to restore just the NC stack, that wouldn't be hard.
I would also do the storage for NC locally, you can mount another dataset/virtual drive into the instance, but using Openmediavault Samba shares for your data store seems like a recipe for disaster. Samba on Linux is a convenience for interoperability with Windows, but using it as a backend for an app as complicated as NC is really not going to end well. I'm surprised you get it to work for more than a few hours, and I could see it losing data so easily.