database sharding, when does it really become necessary

hey everyone, i've been running into scaling issues with a rapidly growing application and the performance of our primary relational database is starting to take a hit. we're not quite at the scale where a distributed database seems like the only option, but single-node scaling is hitting its limits. i'm starting to consider database sharding as a potential solution. for those who have implemented sharding, at what point did you decide it was the right move? what were the biggest challenges, and what strategies did you find most effective for managing shard keys and rebalancing