when do you think its time to shard your database

been wrestling with scaling a monolithic app lately and sharding is on the table. We're hitting some serious read/write bottlenecks that simple indexing and read replicas arent fixing anymore. For those whove been there, at what point did you decide sharding was the right, and often complex, move? What were the main pain points that pushed you over the edge. Curious about the trade-offs you encountered with sharding versus other scaling strategies at that stage.