Is there any macOS Shortcut which exports all reminders in Reminders app to text/ CSV file, and another shortcut which imports all reminders from a CSV file to macOS Reminders app?
A shortcut could be written for a CSV format for basic coverage, but this is unlikely to realistically cover subtasks, images, etc. You would probably want a more sophisticated file format to manage Reminders really as I think natively they are held in a database rather than a flat file.
What this poses though is (given iCloud backups and the like), what your scenario to require this is.