Essentially, the only backup needed of Nextcloud is the database and data which is covered by the respective charms used.
Database backup
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.
Primary storage/data
See the documentation for your selected primary storage charms.
Configuration
The nextcloud instances doesn’t need backups since they will be
identically setup as part of adding or removing units.