2013 Mac Pro always hangs on shutdown

I have a 2013 MacPro. 64 GB of RAM.

The system will not properly shutdown. I have to force it to quit by holding down the power button.

In troubleshooting this problem I have disconnected all external peripherals, all external USB hubs, etc and done a Safe Boot. The system still hangs when I attempt to shut down under this condition. The only thing plugged into the computer at this point are the external display and the internet connection (ethernet, not WiFi).

The shutdown process seems to progress normally. The Finder quits, I have only the black desktop. At some point, seconds, not minutes after the screen is completely black, the arrow cursor moves from wherever it was to the upper left corner of the display - where the Apple icon would be on the menu bar. And then it hangs. As in hangs overnight.

I freely admit that the new logging system is utterly opaque to me. I no longer know what to look for in the logs to understand what might be happening or if the shutdown hangs after the logging system has finished.

Thoughts on how to proceed?

Thank you.