While this isn't specifically for FoundationDB, I wrote this post many years ago about how to implement (secondary) indexes on top of a key-value store. [0]
Interesting that you choose to do it manually. Do you do this as a transaction so that the indexes are always updated. If not, how do you handle partial failures?
Of course you update indexes in a transaction. Doing this manually has a lot of advantages, some of which I outlined in my other comments in this thread.