When should you ditch that one giant database for something else?

weve been running our application on a single PostgreSQL instance for a few years now, and it's been serving us well. But as we scale, i'm starting to feel the pressure. Read replicas are helping, but write contention is becoming a real concern. Weve looked at vertical scaling, but the cost is becoming prohibitive. Has anyone here faced a similar situation? What were the signs that told you it was time to consider alternatives like sharding or even a completely different database technology? What were the biggest challenges you encountered during the migration?