For other users who want to create new company files without having to log into an Intuit account, I was able to do this with an older Qb version (QB 2019 for Mac - which fortunately I still have a working copy of): created a new company file called "Blank Co" which I'll just copy now and modify each time I need to set up any new files.

Obviously this isn't a fantastic solution to the general issue - just mentioning it in case it's helpful to anyone else.

And Intuit - please take heed of your frustrated customers here and retract the tentacles of needless control !!