I’m a big fan of TrueNAS and Proxmox and I think OMV will be great for you.
In the order you asked:
I think OMV is a decent choice, but there isn’t really a bad choice, just better fits for personal tastes.
The upside compared to vanilla Debian or Ubuntu is a solid web-UI for management (though you could get that in the form of Cockpit) and a complete system philosophy. The downside is less flexibility. Any system someone else makes locks you into doing things their way.
If you don’t have a desire to run VMs or set up clusters, or have by-default ZFS on root, you won’t be missing anything.