Best cloud/sync solution

[jawsthemeswimming428]

I have a TS-230 and am looking for some ideas on a good solution for my needs. I'd like the following:

1. A dropbox like file sync experience. I want the files to live on my NAS and only be stored/downloaded when accessed. Then, after changes are made, the new file is saved to my NAS and not on the device. However, I want to be able to view the file structure on my remote device.

2. Ability to give others access to the files with this solution.

3. Mobile solution available to do everything in #1 above.

I currently have QVPN set up and I am accessing files that way. I'd prefer no VPN and a good app to do this. Using QVPN makes it harder to share files when remote and on mobile. A few options I have seen that I'm interested in are Seafile, Resilio sync, Syncthing. I know QSync is there but I'm not sure if that has the features I want? Any thoughts/suggestions would be great. Thanks!