@ahaw021 - Thanks for your response. Upon further reading I have found an slight contradiction that was actually contained in your link (as mentioned by @schoen). https://www.ssh.com/ssh/host-key - “Host Certificates”
OpenSSH does use Public key encryption keys as “host certificates” however they are not the X.509 standard and the Let’s Encrypt keys will not be able to be used (OpenSSH uses another format for signing keys).
It’s a shame because, it would seem to me, if the TECTIASSH ssh method was used, the Letsencrypt validated host might have been used as a signing CA (given we have both Public and Private Keys) we could have registed our own host as the local CA cert with a complete chain of authority. Full certification with a Certificate Authority for SSH. Seems we’re all waiting for https://tools.ietf.org/html/rfc4255 to be solved…