Tarrasque
You can easily extend the list of domains for an existing certificate with the certbot binary: https://certbot.eff.org/docs/using.html#re-creating-and-updating-existing-certificates
We have an open request to allow adding multiple and wildcard domains, but this is not a short-term task on my plan, so might take a while if not someone else finds some time to contribute.
But as dietpi-letsencrypt uses the regular certbot binary, one can adjust everything as above.