Does it terminate existing connections and re-handshake when renewing the cert then? Or does it potentially hold many keys in memory for existing sessions? IIRC a TLS session can potentially last for a long time and 0-RTT depends on it being the same key, right? Couldn't find any answers in the docs: https://caddyserver.com/docs/caddyfile/directives/tls#reuse_...

The renewal of the key does not affect existing connections/sessions because they (the sessions) don't use the key directly. The private key is only used in the beginning to agree on the symmetric key, then the symmetric key is used from there onwards.