when should you consider sharding your database?

I've been thinking a lot about database scaling lately, especially as application loads grow. We've hit a point where a single primary database instance is starting to feel like a bottleneck for both reads and writes. I know replication helps with read scaling, but writes are still a challenge. Sharding seems like the next logical step, but it also introduces a lot of complexity around data distribution, rebalancing, and cross-shard queries. What are your experiences with sharding? At what point did you decide it was necessary, and what were the biggest hurdles you encountered? Any strategies for making the transition smoother?