Dear Gnome,
I really love your system, yet as a connected user who recently tried your "online-account" i have a feedback to do with the hope you actually make a change or two
First can you change the way on how "file" work to allow a sync instead of a direct connection who force us to use third party app
Second for services that support it, can you please move from the old as hell mechanisme of app password by the user and use the modern way browser)
I think OP wants authentication to services to happen via OAuth (or is it called something else), where you get redirected to a web interface, sign in with your Master password and grant the Gnome app access to the service. As opposed to having to create an app password in e.g. Google web interface and using that for the Gnome app
But I can't see any mention of Nextcloud either...? I took your post to mean any of the available online accounts, as you're didn't mention one specifically. Google was just an example
Sync needs algorithms (what to do with conflicts), which depend on the vendor, so built-in support for Nextcloud sync without their client installed isn't going to happen.