Archive first ... those would be full backups of courses ... users, students, their assignments, etc. Then copy those archives off onto another server for storage.
Think you need to read up on 'reset' of courses.
https://docs.moodle.org/33/en/Reset_course
In the newer versions of Moodle when looking at what to reset in each course, turn down/open each option - reason - easy to miss the student options to remove not only students but their work while resetting a course.
Do one or two like that, then see the results. Once you get that down the way you want, it's easier going forward.
'spirit of sharing', Ken