If your courses are in Legacy, they will forever be in legacy on backup and restore - unless one edited the moodle_backup.xml and files.xml in an un-archived backup, re-compressed (properly), then restore.
Maybe a slightly different flow is needed if there are other courses like this one.
At end of this year, backup the course for archival purposes ... could restore to dev using Moosh if need be. I'd do that via command line backup from the script in admin/cli/ and point the backup to go to a unirepo that dev can reach/see.
Instead of backup and restore, do 'reset' of the current course. Choose options to remove students and their assignments, quiz attempts, grades ... anything related to student data ... basically roll it back to like it was before students were enrolled. Backup the course again via command line. That's a base line course then one could work on in spare time to rid of Legacy, etc., etc...
Also consider breaking up a full year course that you know by the end of the year reaches 'critical mass' when it comes to backups/restores. Make them semester courses even though the real class is a full year. I've had to do that with courses like 'Digital Photography' and "American Sign Language" (the second used video for everything! The best course I've ever seen when considering objectives of the course and appropriate use of technology! Students had a blast with it and learned along the way!!!).
Anyway ... 2 cents worth ...
'spirit of sharing', Ken
And please let me know if you need that course I restored on the other box. I need to remove it.