If you want to continue using acme.sh + Let's Encrypt, this command will suffice:
acme.sh --set-default-ca --server letsencrypt
Note that this will not change CA's for existing certificates, so any existing ZeroSSL (or Let's Encrypt) certificate will continue to be renewed with that CA. [There was a bug on master that this wouldn't hold for very very old acme.sh configs, but I believe this is fixed now]
ZeroSSL does not document any (public) staging enviroment at this time.