For windoze target hosts, I have found the Cygwin implementation of the OpenSSH server to be relatively well-documented and straightforward to setup.

Yes it is a bit "heavy", but note you don't have to do a full registry-based install on every client. Just get it a minimal setup running in "portable mode" and keep the whole X:\Cygwin tree in sync with rsync or whatever you already use.

However from a security POV, the SOP is to let Cygwin's normal OpenSSH setup routine create the special full-rights-to-backup user, password issues, so for small environments you probably should just go ahead and run setup on each client anyway, you can still sync later updates.

It's actually nice to know you've got sshd available on your clients for other purposes, especially to ensure good security for FOSS tools that don't know from domain-based tools.

And of course the other platforms have the ssh server enabled out of the box. . .