For what it's worth, Traefik does allow you to change the server you talk to (it's just the staging server for LE they use in the example): https://doc.traefik.io/traefik/v2.0/https/acme/#caserverand

A quick google does suggest that at least one person has tried using ZeroSSL with Traefik: https://spad.uk/get-free-zerossl-certs-using-traefik/