Research if it's possible to fetch existing certificates
Ideally, when a cert is missing (but private key exists), it would be useful to detect if a valid cert with the same alt-names & features already exists (as issued by a master or other follower), and fetch it rather than create a new one.
See https://community.letsencrypt.org/t/how-to-revoke-a-cert-i-did-not-issue/58306/2 about fetching existing certs by serial number.
Don't know about getting a list of all certs, possibly from a CT log?