Essentially, the only backup needed of Nextcloud is the database and data which is covered by the respective charms used.
Read in the Deploy Postgresql using Charmhub - The Open Operator Collection charm documentation on how to backup the database for Nextcloud as this is covered by that charm.
See the documentation for your selected primary storage charms.
The nextcloud instances doesn’t need backups since they will be
identically setup as part of adding or removing units.