You can't reasonably expect most people who would register to a service over ssh and make backups with cUrl in a shell script to update their known_hosts file?

The instructions on the main page would change from:

  ssh register@ui.baxx.dev

to something like

  curl https://ui.baxx.dev/ssh_keys -o /tmp/baxx_ssh_keys
  cat /tmp/baxx_ssh_keys
  cat /tmp/baxx_ssh_keys >> ~/.ssh/known_hosts
  rm /tmp/baxx_ssh_keys
  ssh register@ui.baxx.dev

or in one line:

  curl https://ssh_key.baxx.dev >> ~/.ssh/known_hosts && ssh register@ui.baxx.dev