The "simplest" gains come from adding an in memory cache, we include Redis for this and some apps work really well just leaving the DB where it is, caching aggressively, and running close to users: https://fly.io/docs/redis/

Read only replicas are a great first step for most applications. I'd probably do caching first, then replicas (which are kind of like caching).

Region specific persistence is one way to improve write latency, and I think the simplest for most apps. We've experimented with CockroachDB for this (it keeps ranges of rows where they're most busy), and you can actually deploy MongoDB this way.