GNOME has a feature for managing online accounts in one place ( GnomeOnlineAccounts | GOA).
In it’s current state Nextcloud is supported and it can read calendar, contacts and documents (as in providing a quick mount option in the file explorer): https://wiki.gnome.org/Projects/GnomeOnlineAccounts/Providers
This way account credentials can be shared and are configured and managed in one place where they can be requested by an application. The communication is done via D-Bus.
Currently I have to configure my accounts in two places, the nextcloud-client and in GOA, to get a Nextcloud server tightly integrated into my system and while the passwords end up in the same place (gnome-keyring) it’d be more user friendly to share this from a common ground instead of having to configure this twice for each Nextcloud account.
I can see the need for that as I’m currently struggling with the same issue. The GOA integration will pull in address book and calendar and such, the client will sync files.
However, if gnome GOA would be able to sync files as well (as I believe would be the best solution), there wouldn’t be any need to use the client at all. Therefore, this issue should be raised with Gnome, where this would be implemented, not with the nextcloud project.
GOA manages no sync. It provides credentials and single logon.
e.g. for Mail it’s up to the client (evolution) to fetch and store a box.
That’s out of scope of GOA. Think about this like the Accounts settings on an Android phone.