Yeah, you can use any ACME-compatible CAs with Caddy.
Buypass works. I’ve tested that.
Google Trust Services also works.
Some CAs will require an External Account Binding (EAB) that you’ll have to add to your config first.
But yeah, you can absolutely use any ACME CAs you’d like.
What issues are you having with ZeroSSL, exactly? Anything like this?