Running your own Postgres on your own server — implementing and testing your own backups, optimizing your own filesystem, managing encryption keys, managing upgrades, etc — is not simpler than using Google Cloud SQL, which does all of this for you at an SLA you will not be able to achieve if you will be focusing on your business, which is what you should do as a startup.
Certainly you should not be running your own K8S cluster, but using Google Cloud Run is simpler than keeping your own server running. Even using Google Cloud Kubernetes Engine with autopilot is simpler than keeping your own server running.