You might want to have an RDBMS for config and logging, so you can report what sent how much how often when.

Many such systems let correspondents drop files in write-only dirs, or remove the files once they are valid and complete or after a time.

ssh gives remote login or command execution and names the whole facility, SCP moves files, tunnel allows remote connections, and sftp is just an ftp-like front end for ssh file transfer. This is not a tunnel sort of application. Tunneling is just what you want to prevent!

Yes, store and forward adds security. Also, you can online/offline archive copies for a while in case the lose them, and need a rerun (or to fulfill a court order ). Compression in a nice economy of disk space, bzip2 or the like, after a few days, and zip archiving of small files saves disk pages and inodes.

You can poll the folders easily enough using your code. You are actually reinventing the wheel, so snoop around at things like Tidal to see what features you want. You might integrate a secure web service for administration, reports, operator manual activities and even external user status reports. It could all be scripted, using command line tools like isql for db access, or PERL is nice both for DB and web, or JAVA.