I have never used the Flink Cassandra Sink so this may or may not work, but have you tried creating your own custom retry policy?
https://docs.datastax.com/en/developer/java-driver/3.4/manual/retries/ Returning an ignore when onWriteTimeout -- Sent from: http://apache-flink-user-mailing-list-archive.2336050.n4.nabble.com/
