does calendar, contacts, photo management, maps, phone tracker, music player, forms (like google forms), todo lists, news aggregator (RSS), integration with Jitsi for web conferencing.

And for all of those things, my desktop and my phone have specialized applications which work better than Nextcloud in every way and do not add a huge attack vector to my personal infrastructure.

SFTP may not be the right tool for the job here, but with something like Syncthing it is infinitely easier to synchronize the data between the devices and the specialized apps work directly on the data, than forcing all different types of data through the same single type of application. There is no such thing as “dumb clients” anymore, why not leverage that?