"500 KB/s workload should not use Kafka" - yyyy!!! indeed, I'm running 5MBps logging system through a single node RDS instance costing <$1000/mon (plus 2x for failover). There's easily 4-10x headroom for growth by paying AWS more money and 3-5x+ savings by optimizing the data structure.
I've always said, don't even think about Kafka until you're into MiB/s territory.
It's a complex piece of software that solves a complex problem, but there's many trade-offs, so only use it when you need to.