Ah yes, nextcloud would be important to back up. Audiobooks you can probably re-aquire but if people are storing personal stuff on Nextcloud you will want to make sure it's backed up. A common strategy is the 3 2 1 backup strategy. Have three copies of your data, two on different media, and one offsite.

It sounds like you have your original copy and your offsite copy now set up. I'd recommend having an extra backup, a backup of the nextcloud data on a different hard drive on the same machine (or another machine - but this copy should be on a different hard drive to the original copy). Personally I don't do anything fancy, I just run a cron job to bring down nextcloud (so nothing changes), and to run a script to zip up the nextcloud data (I use bind mounts for volumes so everything sites nicely in a nextcloud folder, including docker-compose.yml file, database, and nextcloud data), then I copy that zipped file to my backup drive with the date in the file name so I have multiple copies. I run this each night scheduled in the middle of the night, then have another script that trims the backups to keep 14 days of backups then a monthly backup indefinitely (always first of the month, for simplicity).