I was able to get the program to work, at least via command line.  The comments were helpful, especially stripping the program down to the minimum needed to show the issue.  An explanation is below.

Root cause:  I believe the culprit was a missing tk-tools library.  Tk, included by default, allowed the GUI to be built and loaded but not executed.  You don't have to import tk-tools and I never received an error message that the functionality was missing. The "local" or default Python instance does not include tk-tools.

Method:  Retracing my steps using the history command I noticed while I entered the python -m venv command, I didn’t follow up with source activate.  This meant I was still using the “local” Python instance, and libraries.  It became apparent when I added-back some code deleted for this question but received an error that the referenced library was missing.  The original script was created inside the venv and included tk-tools and other libraries but most of my subsequent development didn't activate the environment. Some libraries were installed on both.

Thonny:  Like the command line, Thonny defaulted to the “local” python instance.  Dummy me, I assumed since I had a venv structure, Thonny would have used it to execute the script.  There are internet pages providing instruction to configure Thonny to use a venv but those instructions didn’t match the screens of my version.  Given I had the command line working I didn’t pursue further.