My ballpark figure would be 100-150W idle for the dual xeon setup. Taking out one CPU would probably drop you 30-40W.

Modern Ryzen still isn't very efficient at idle (at least not for me).

If you're really concerned about power cost: get a used SFF business PC like a Prodesk G4 or something. That'll get you idle draw around 10-15W (and chances are the CPU actually beats the Xeon at most tasks).

Nice troubleshooting steps, even though you're (still) mixing up some units ;)

I'm guessing something in the network card or its driver, or an issue in your network (congestion, flooding?).

Any chance you can manually reload/restart the networking system? I don't know about OMV but on a linux distro it's usually a matter of restarting networking/NetworkManager etc.