Been down since the 9th of July for me. The trends dashboard has always been terrible. I really don't know how hard it can be to get this to work. What are we paying our subscription and apple fees for?

Thanks for the ideas, all. The OpenCore system mentioned on MacRumors looks like my best bet since the machine is only one year past Apple's obsolescence barrier. It looks relatively (ho ho) straightforward to install Monterey - if I succeed then it might let me carry on for a while with the same perfectly good machine. I will see.