I have finally managed to install Nextcloud to my freenas server, and while I still have some figuring things out to do, it generally works quite well.
My basic Wish was not so much to just sync photos FROM my phone to the server to provide the backup, but rather to also push new photos TO my android tablet, which so far I havent figured how to do automatically. The more laborious way, employed at the moment is to manually transfer new photos from phone to the tablet, so I really need an automates solution that can also write to an SD card under android.
Did I misunderstand the syncing options of nextcloud?
By installing the Nextcloud app files are primarily synced from your phone to Nextcloud. You can access all files afterward by opening the Nextcloud app and navigate to the destination folder to which you’ve synced the files and open it from there.
If you always want to sync files back to your phone without access the files online, you need to use e.g. the Foldersync-App which allows to sync files from a Nextcloud WebDAV share back to your device.