I’m looking for a way to incrementally back up the NextCloud database (MariaDB in my case) and the files. Best would be to have a full backup at the beginning of the week and incremental ones for the rest of the week. Has anyone a similar backup plan in place?
Yes. If you do not use a snapshot-able filesystem like BTRFS or OpenZFS, then this might be an idea:
Then you make a rolling 7 days of age snapshots, and one full rsync backup once a week. However you could also just do a daily delta sync using rsync, as well as snapshotting. No need to do a weekly full backup, if you do a daily - or even an hourly - delta sync with rsync.
I use the Veeam agent for Linux. The free version is kind of feature limited but it does daily incrementals, supports SMB and NFS natively, and is easy to set up on headless systems.