@darius I know you've found your workflow solution already (and you have been incredibly patient communicating that to everyone!) so I'm not offering advice, but: is there any chance you'd be willing and able to share the version and configuration of the sshd on the listening side? I would really love to try to dig into the exact underlying issue(s) here, even if just for my own edification!
@gnomon `sshd -v` gives me "OpenSSH_7.6p1 Ubuntu-4ubuntu0.3, OpenSSL 1.0.2n 7 Dec 2017". I don't know if that's what you are looking for re: version, but it's the same as a different server that was accepting connections just fine. The /etc/ssh/sshd_config on both servers, working and not, is identical too. Is there another file I should be looking at?