How do I sync shared folders on a server over the web or the Dropbox client for the server?

Question

NAS

Initial data:

• Windows Server 2008 R2 as a file server and terminal server (remote desktop).
  
• RDP clients (work over the Internet) under Windows
  
• File server clients (Samba) under Windows on the same local network as the server
  
• Clients who want to access shared files on the server (1 shared folder) but don't want to access RDP to just work with files on the server. Who works from where, they are not in the same local network, they have nothing "in common".

Which options are NOT considered and why not:

• Provide first VPN or RDP access to the server. Samba under a VPN on a weak link will be a pain. On RDP, people do not want to go to the terminal server every time to get or pull a file to a shared folder.
  
• Raise an FTP server on the file server . The FTP client frustrates people, everyone wants "just a folder" or a network drive.

What options are possible in principle, but there are nuances:

• Third party cloud service like Dropbox or OneDrive. The idea is good for everyone, we start one common Dropbox, Google Drive or at least Yandex.Disk account, put each "lazy client" on the client's computer, prescribe one login and password ... The client is fine, but we run into a terminal server. How to make sure that each RDP account on the server does not have to install this client? It hangs in the memory of each user, and tries to simultaneously overwrite and hash the shared folder on behalf of each terminal user... The folder is the same for all, the data is the same for all, why do this 10 times on behalf of each user???
  I can't figure out how to make it work on behalf of the system (as a system service). Create a special user for this, register it in Autorun, specify a file server subdirectory with full rights "for everyone" as a synchronized folder and keep it permanently logged in on the server?
  
• WebDAV. Seems like what you need. But to lift IIS, to do separate authorization and to expose the rights separately only for access from the outside to one folder?