Ok, I think I mis-understood your intent a little.
This should be what you need:
https://rclone.org/docs/#backend-path-to-dir
rclone copy --sftp-host example.com :sftp:path/to/dir /tmp/dir
This will basically define a SFTP-remote in-line with the command. Note the very spesifc syntax there in :sftp: and that this is a protocol (or I guess more spesifically a remote-type), not a remote name.
That example will only work on a default port, with anonymous login ect.
You probably also need to add:
--sftp-user
--sftp-pass
--sftp-port (maybe not..)
(there's an optional flag to set password to be prompted instead if you want that, see docs)
This should then allow you to do rclone operations directly to an existing sftp-server, and it's a single command. Mark the answer if this solves it for you 