How to offload complex Firestore queries into Firebase Cloud Functions and parse results on iOS/Swift client?

So currently, my client (iOS/Swift) is fetching documents by filtering on a whereField(...) and then further filtering down the results based on a few other fields. This means that some objects are fetched, then discarded, wasting precious resourced and bandwidth!

Can I do this complex filtering on a firestore cloud function than somehow parse the filtered results into the same firestore object on the client? To be clear, I know how to write cloud functions that fetch and return documents, I am more interested to understand the transport method to the client.

Code snippet to provide some context:

static func unmatchedChatsHelper(cursor: Any?, promise: @escaping (Result<UnmatchedChatsCursorResponse, Never>) -> Void) {
    let earliestEventTime = Date()
    // (1) Filtering done on backend.
    var query = Firestore.firestore().collection("chats")
        .whereField("eventTime", isGreaterThan: earliestEventTime)
        .order(by: "eventTime")
        .order(by: "createdAt", descending: true)
    if let cursor = cursor as? DocumentSnapshot {
        query = query.start(afterDocument: cursor)
    }
    query.getDocuments { (querySnapshot, error) in
        guard let querySnapshot = querySnapshot else {
            fatalError("Query snapshot failed: \(error?.localizedDescription ?? "no error")")
        }
        let transformedModels = querySnapshot.documents.compactMap { (chatQueryDocumentSnapshot) -> ChatModelFirebase? in
            do {
                return try chatQueryDocumentSnapshot.data(as: ChatModelFirebase.self)!
            } catch let error {
                logEvent(AnalyticsEventDidDropDocument, [
                    "document_id": chatQueryDocumentSnapshot.documentID
                ], error)
                return nil
            }
        }
        let me = Auth.auth().currentUser!.uid
        // (2) Filtering done on client! This means some chats are retrieved unnecessarily. 
        let unmatchedChats = transformedModels.filter {
            !$0.participants.contains(me)
                && $0.numParticipants < 6
                && !$0.declinedParticipants.contains(me)
                && $0.isDebug == DebugGating.isDebugBuild()
        }
        let cursor = querySnapshot.documents.last
        promise(.success(UnmatchedChatsCursorResponse(models: unmatchedChats, cursor: cursor as Any)))
    }
}

I have three options:

1. [My Choice/Best Option] Same as above, but use Firestore.Decoder to parse results ChatModelFirebase.
2. [Parsing Logic is Dangerous] Do the query on a cloud function, return JSON, and manually parse the results into ChatModelFirebase.
3. [Too Costly To Do] Update my firestore document structure for all queries to be done via the whereField(..) but this is complex since whereField is limited in its options.

Anything else I a missing? Is there a better way to do this?