It's really important to manage versioned schema for events and defining rules around evolution of schemas, hopefully with tooling support like via Avro's Schema Registry or Protobuf's forward and backwards compatibility guarantees.
No, it's really not a high number. It's right on the money for 3 years in-the-trenches experience making the move into consultancy -- around $130-140/hr billable, which is what I was billed out at in the mid 2000s as a .NET consultant.
Getting high hourly rates isn't the hard part. It's maintaining consistent work across multiple clients while taking the time to market yourself and negotiate contracts. That's not an easy task for someone who has never worked on their own before. Unless it's an unusual circumstance (like he had one client the whole time), I think averaging 20 billables a week in the first year is pretty impressive for someone to do.