Relying on an eventual consistency database sounds like a huge challenge in terms of getting absolute consistency between Cassandra and Kafka. There seems to be many edge cases that could cause the data in Kafka to get out of sync, it doesn't sound very reliable. Especially if you want to rely on Kafka for event sourcing later on, which is sounds like you're planning on in the future. For a payments company that sounds very risky.
What happens if you write a row to Cassandra but that row is invalidated because of failed writes to the other nodes in the cluster (ex. the write was QUORUM and it succeeded on the node that you're reading from but failed on all other nodes). In Cassandra, when an error occurs, there are no rollbacks, and the row gets read-repaired. It sounds like that row will get outputted to Kafka. How do you "disappear" data written into Kafka if it gets read-repaired?
What happens if you write a row to Cassandra but that row is invalidated because of failed writes to the other nodes in the cluster (ex. the write was QUORUM and it succeeded on the node that you're reading from but failed on all other nodes). In Cassandra, when an error occurs, there are no rollbacks, and the row gets read-repaired. It sounds like that row will get outputted to Kafka. How do you "disappear" data written into Kafka if it gets read-repaired?