Since my initial post I have tried 3 more times to update to Windows 10 2004

First time before the Sept cumulative update I got the same as previously with Google Synch “Paused” and periodically asking for my Password on MS Outlook 2016

Second time after Sept Cumulative update the same thing as before with an added complication.
After several hours when I tried to compose an email that particular window got very unstable and I could not even select any contacts in the “To:”
Also unable to write anything in the “Subject:”
I did find that if I changed the Chrome “Startup” tab to “google” instead of “Met Service” it was ok on “Synch” for 12+ hours, and I thought I had resolved the Chrome issue but no the issue came back to haunt me LOL

Third Time I cloned the NVME back onto the old HDD and also created a new Local Account before updating to 2004.
Now I know why I changed to NVME, the HDD was so slow by comparison LOL.

Anyway I had the same issues as in the first and second tries.
When I switched over to the new Local Account (still logged into the original Local Account and unstable Compose email window) the new Local Account Compose Email window was stable.
Also the Chrome Synch on the new account was OK.

This suggests to me there is an issue with the original Local Account rather than with Windows itself.

If I was to do a clean install on that spare HDD using Windows 10 Pro (like on the NVME) to try it and see what happens would I have to get a new windows licence?

I would try changing the BIOS order between the NVME and HDD to see what happens (note the Data is on a separate SSD so that is not an issue)
If I was to add MS Office 2016 to that I know I get 30 days before I need to activate that.
I would add Chrome to the HDD as well