So.. I wrote a quick how to on setting up a backup MX server with #OpenBSD and #OpenSMTPD since the only one I could find was a bit dated. I wrote it for gemini and did a hasty conversion to html manually. It isn't pretty but it gets the job done:

https://sdcch.org/opensmtpd-backup-mx.html

gemini://sdcch.org/opensmtpd-backup-mx/

Thank you @solene for giving it a once over, your blog inspired me to do this!