Unless something changed in the last few weeks, Android app can't do bidirectional folder sync like you're used to from Google drive, Dropbox or one drive. You can mark a folder for "sync", but try copying something into it, it changing an existing file: no change on the server. I think you can manually open the app, navigate to the folder and open the menu to "sync", but what the hell is the point in having it marked to be synced then? I wanted to use it for my keepass DB for example, where that would be a requirement.

Yes, updates are a nightmare. I'm not on docker (also not sure if better or worse) and every 2nd update sometime significantly breaks that I have to fix manually. Last time, the "group folders" plugin (official, from the Nextcloud devs) broke it so bad that Nextcloud wouldn't even start. I had to go into maintenance mode, disable the plug-in ("app" I guess it's the term?), update it from console, re-enable it, disable maintenance mode. Not pictured: finding out what the god damn problem was this time. I've just about had it...