Start Simple, Not Scalable
Choose boring technology. You do not need Kubernetes for 100 users. Monoliths are fine.
Database Choices
PostgreSQL can handle almost everything. Only shard when you actually need to.
API Design
REST is fine. GraphQL is fine. Consistency matters more than the paradigm.
Monitoring and Observability
Build logging and monitoring in from day one. You cannot fix what you cannot measure.
When to Break the Monolith
Wait for team size to force it, not user count. Conway's Law is real.