when should you shard your database

hey folks, i've been running into some scaling issues lately with our app, it's getting bigger. we're using a single postgresql instance right now, and writes are becoming a bottleneck. we've already optimized our queries, added indexes, and are looking at read replicas. but for the long term, we might need to think about sharding. for anyone who's dealt with this, what were the main signs that told you it was time to really think about sharding. and what were the toughest parts of making that change