UPDATE: I finally had the time so I decided to go back to a Checkpoint in Hyper-V of the server from August 2020. I loaded up the checkpoint and it was running as expected with no issues loading the Zentyal GUI after boot.
I then used the webGUI to check for updates, of which there were many, and I applied them all. After they were all applied, there was an error installing one or more packages, but I rebooted the server anyway. After 35 minutes waiting for the server to load there was still a black screen. So, I restored the checkpoint from August... again and confirmed it was working, GUI and all. I then downloaded and applied all of the updates, received the package install error again, went into the terminal and fixed the package installs, the rebooted the server and it booted with the original GUI loading problem I describe in my post above.
So, it is confirmed that an update broke the Zentyal GUI on my install. I am now trying to install one update package at a time to see if I can narrow down which one caused the issue, but it is a long process if I am to test each by rebooting after every package install.