I might not be 100% correct on this, but it seems like even Algolia keeps all records in a single host and uses the other 2 hosts for only high availability.
Having said that, exploring a FoundationDB integration is definitely an interesting idea. However, quite a lot of use cases can be served perfectly well with a simple master+slave set-up, so my primary focus is on that until there is enough demand for horizontal scalability. For e.g. Typesense is not a great fit for things like log data that typically need large amounts of storage.
Have you considered using FoundationDB as a storage layer to match that feature?
Disclaimer: I have no idea how to build a distributed search engine.