If it can help you and the cause (cinnamon, debian, ?) I am happy to continue with experiments as you have suggested. Now I can install almost any configuration easily, and when there is a freeze, I can get diagnostics from the ssh terminal.
What I lack is any experience in what to look for. I am unclear about which system logs are likely to report relevant info, and where they are located
One thing I did not explain clearly is that when I say cinnamon freezes, the pointer on the screen still moves with the mouse motion. There is however no inputs accepted: mouse clicks are ignored as well as any kbd input.
Another question relates to which cinnamon configuration is involved. There are 3 options: cinnamon, cinnamon software or cinnamon wayland experimental. But is there any way to tell which configuration has been selected once cinnamon is up and before it freezes? I have not been able to distinguish any difference in response no matter which of the 3 options I selected
Finally, if we just let this go, how would I find out in future if anyone thinks they have fixed the problem?