Thanks Goose66 for the pointer. Moving to Python 3.7.3 fixed the problem, MyQ now works fine.
I scratch installed Rasbian Buster, PG2, and implementing the CipherString fix
was able to get PG2 to run. (PG2 won't install on either 32, or 64, bit Bullseye). Buster has Python 3.7.3 by default.
Retiring as an Infrastructure Architect, after 35 years in the IT vendor industry, it was not my experience that software required proprietary hardware to be stable. When platforms, like PG, clearly articulate the operational requirements (for me in this case), as well as (don't shoot the messenger) development constraints for its plugins, things will usually run OK.
The issue, with the rat's nest PG support turned into, was that the PG middle-ware didn't have the leverage and/or bandwidth to dictate anything to anybody. UD picking up support for PG will resolve this.
It's unfortunate that UD won't at least provide a github for linux users for PG3. They would simply need to document exactly what the runtime environment must be. It seems likely their new hardware is running some flavor of linux. They wouldn't necessarily have to take support calls for it (Similar to Home Assistant where if you aren't running their kernel then don't expect support).
Their decision is understandable given their income model is based on selling new hardware. Their post-installed support has always been five star, and they give away major software rewrites/enhancements for free. Have to pay the bills somehow. Personally, I prefer the 'pay for support' model, for critical infrastructure. I would have happily kicked in 20-50$ for the 5.x rewrite, or pay (say) 50$ for supporting PG3 users on a linux os.
It's slightly disingenuous to say the ISY994 will be supported even though it's installed base can't run Polyglot. If I can't run PG then I will need new UD hardware, or move everything over to another of my RPis that runs Home Assistant.