You've posted in backup and restore, yet your explanation leads me to do something it is said one shouldn't do .... answer a question with a question ... but am gonna.
If you would, please, extend out to the year 2030 what has transpired for just one of your courses and if you'd care to guess, what would be the size of that course backup file?
I have an un-offical term for such courses ... a 'forever' course. Is that what you wanting to do? You say courses should retain data ... what data? All users, their assignments, the quizzes and grades, their forum postings ...
The courses you do have and want to do this way ... you say for the purpose of retention of data ... are you under some sort of thing that has such a rule ... like in Texas K12 - student graduates and ISD is expected to retain all records for that student for up to 5 years. Wow! Think about that a minute IF ... IF that student started in K and graduated and in ALL ... ALL of their years in school a moodle was a major component - thus ALL of their work in Moodle had to be retained!
Are we really posting in the 'best' forum for the question?
How about: Teaching with Moodle?
https://moodle.org/mod/forum/view.php?f=24
By posting where you did, are we already thinking massive storage site ... so large that one will eventually have dedicated web server(s) - that's plural so load balancer - dedicated DB server - dedicated storage for moodledata/filedir/
Not trying to be difficult here ...
'SoS', Ken