How big do you think is "big"? I run a system that handles about 3k requests per second (~1:100 writes to reads). No caching, no Redis, just Postgres. One master and a hot standby for failover. It sits around 25% CPU, and most of that is from opening and closing connections rather than queries.
There's an important lesson in avoiding complexity for speculative reasons. Unless you have data that suggests more components materially improve the system, save yourself the heartache and build for the simplest system that meets your needs. You've built a dump truck when you probably only need a pickup truck, and while you might need more than a pickup truck someday, you're paying dump truck maintenance costs in the meantime (and your future needs probably look more like a King Ranch than a dump truck anyway).