I can totally understand your frustration here but what @ikakavas wrote is the truth. If a 3rd party software does not support a mandatory feature, then you need to ask this 3rd party to add it.
The functionality to write data to a secured Elasticsearch instance is not yet implemented.
In the latest Kafka elasticsearch connector version (5.1.2), they seemed to have made some progress though. See https://docs.confluent.io/current/connect/kafka-connect-elasticsearch/index.html#security
The Elasticsearch connector can write data to a secure Elasticsearch cluster that supports basic authentication by setting the
connection.usernameandconnection.passwordconfiguration properties.
Do you have a chance to test it? I'm not sure if it supports https though. But for sure the right answer is to be found in Kafka mailing list. I don't think there is a way to disable TLS once security is activated on an elasticsearch cluster running 6.6+...