So, I’m a bit late to the party, but i haven’t messed with Nextcloud since I asked a similar question and was told basically “yeah, it should do that” (Mobile App Functionality: Sync Folders) and judging from your post here that is apparently not the case (which makes me sad, as I was really hoping to migrate away from Dropbox).
Having said that, I’ve used an app on Android for years called DropSync (by Metactrl, who also make similar apps for Box, Google Drive, OneDrive, something called MEGA, etc). For free it will completely synchronize (two-way) a folder on your Android device with a folder in Dropbox, which is something the native Dropbox app does not do, and my understanding is that no app is allowed to do on iOS unless Apple were to relax restrictions in their API. There isn’t a folder ‘ownership’ that I’m aware of, or maybe it just sortof pretends it ‘owns’ all the folders you configure. The Pro version does the same, but for multiple files, and is well worth the price. I sent an email to Metactrl to see if they might consider making a version for NextCloud, because that would be amazing. But in the meantime, I’m trying out Synchronize Ultimate
In my case, I synchronize the downloads, DCIM and documents folders both between my and my wife’s phones and with a Windows server VM. I wrote a program (running as a Windows service) that renames my pictures and video by timestamp and sorts them into folders by date, and Dropsync takes care of propagating all the changes to all the phones. Anything over two weeks old gets moved to backed-up storage media which is scanned by Plex so we can stream it all to TVs or devices without taking up space. Consequently the files are moved off of our phones at that time. For this to work it’s gotta be full two-way sync in all places, so that’s the one thing holding me up from switching to NextCloud.