I'm not familiar with Barman but if you're looking for a backup software for Cassandra, have a look at Medusa from The Last Pickle -- https://github.com/thelastpickle/cassandra-medusa/wiki.
It's open-source and is also used for https://k8ssandra.io/ -- the platform for deploying Cassandra on Kubernetes with tools for repairs, backups and monitoring built-in. Cheers!
