> If you use Caddy that's even the default behavior - it tries ZeroSSL first and automatically falls back to Let's Encrypt if that fails for whatever reason.

No, that's false. It's the other way around.

“If Caddy cannot get a certificate from Let's Encrypt, it will try with ZeroSSL”. Source: https://caddyserver.com/docs/automatic-https#issuer-fallback

Which makes sense, since the ACME access to ZeroSSL must go through an account created by a manual registration step. Unless the landscape changed very recently, LE is still the only free ACME that does not require registration. Source: https://poshac.me/docs/v4/Guides/ACME-CA-Comparison/#acme-ca...