It seems there is no way to specify the streamType and also provide custom settings to mongo (like the codecs etc.) this is because the method in the com.mongodb.async.client.MongoClients is private:

private static MongoClient create(final MongoClientSettings settings,
                                      @Nullable final MongoDriverInformation mongoDriverInformation,
                                      @Nullable final String requestedStreamType)

So if you pass a ConnectionString you cannot specify codecs. If you pass a MongoClientSettings you can specify codecs but not the requestedStreamType