Does it make more sense to have many collections, with smaller number of entries per collection, or 1 collection with many documents. We are talking 500 thousands documents in total. so 1 collection -> 500k entries many collections -> 1k -> 20k documents we mostly search through 1 collection at a time, if we were to split it up in many collections. Would this bring any search gains, vs the overhead of managing the collections? re-indexing is easier if we have many collections.

That’s a lot of fields! 😄 In your case, I would recommend sharding the data into multiple collections (many collections -> 1k -> 20k documents), because in general the smaller the collection the better performance you can expect.