A startup that outgrows an EC2 server will be making enough money to hire more people to scale the system properly than what was initially designed: trading away everything for development velocity.
Kubernetes is not the right tool for this startup. Kubernetes is what large, old-school non-tech companies use to orchestrate resources, because it’s easier to find someone that “knows k8s” (no one knows k8s unless they’re consulting) than it is to find someone that can build properly distributed systems (in the eyes of whoever is in charge of hiring).