You have it backwards
Cassandra was written at Facebook by Avinash Lakshman, one of the authors of Dynamo at Amazon. DynamoDB is essentially the external version of the in-house Dynamo tool that predates Bezos' famous API directive.
As for ScyllaDB vs. Cassandra, if you are starting from scratch, why would you incur the overhead and GC pauses of Java if you don't have to? There's a reason why Facebook doesn't use Cassandra for anything critical in-house, even though they originally developed it.