Many organizations use JanusGraph on this scale. Insertion of data is slow so you need massive parallel operations to do an entire bulk load overnight. Most people use tools like Apache Spark for this. A useful blog series on this, can be found at (you see, there is a wide community!):
https://www.experoinc.com/post/janusgraph-nuts-and-bolts-part-1-write-performance
https://www.experoinc.com/post/have-you-had-your-janusgraph-tuneup
https://www.scylladb.com/2020/05/14/zeotap-a-graph-of-twenty-billion-ids-built-on-scylla-and-janusgraph/

Of course, you have to monitor the Cassandra and Elasticsearch clusters to check whether they are well balanced and not overloaded. Although JanusGraph can handle some overloading ("TemporaryBackendException") there are limits to this.

If you do not have any Cassandra legacy or experience and with the knowledge that JanusGraph fully supports ScyllaDb, you probably can believe the pitch made by Scylla itself:
https://www.scylladb.com/scylla-vs-cassandra/

Best wishes,    Marc