Now what do you want? Use your wildcard certificates from nginx or have mailcow generate only for itself? If the latter, you have to ENABLE Lets Encrypt, not disable it…
Also you cannot check “force SSL” in your “Edit proxy host” configuration. Otherwise Lets Encrypt will never work. That is one of the problems, if you want a reverse proxy, please stick to its documentation.
To me it seems you don’t really know what to do or are not following the documentation. Read the documentation!
