when does it make sense to shard a database

Hey folks, been thinking about database scaling lately. We've hit a point where a single instance is starting to struggle with load, and sharding seems like the next logical step. But it feels like a pretty significant architectural shift with a lot of complexities around cross-shard queries, rebalancing, and managing multiple database instances. I'm curious, at what point did you all decide to shard your databases? What were the key indicators that pushed you to that decision, and what were the biggest trade-offs you encountered?