Use nested maps to retrieve and save data to Firebase - Flutter

I have three models I'm working with to retrieve and write data to my Cloud Firestore.

//Sales model
class Sales {
  String id;
  String userId;
  SalesDetails salesDetails;
  Timestamp createdAt;
  Timestamp updatedAt;

  Sales();

  Sales.fromMap(Map<String, dynamic> data) {
    id = data['id'];
    userId = data['user_id'];
    salesDetails = SalesDetails.fromMap(data['sales_details']);
    createdAt = data['created_at'];
    updatedAt = data['updated_at'];
  }

  Map<String, dynamic> toMap() {
    return {
      'id': id,
      'user_id': userId,
      'sales_details': salesDetails,
      'created_at': createdAt,
      'updated_at': updatedAt,
    };
  }
}

//SalesDetails model

class SalesDetails {
  String id;
  PurchasedItems purchaseItems;
  String bankTransferSenderName;
  String chequeInfo;
  String creditPayLaterInfo;
  String customerId;
  String customerFirstName;
  String customerLastName;
  double subTotalAmount;
  double totalAmount;
  double taxesDue;
  bool isComplete;
  Timestamp createdAt;
  Timestamp updatedAt;

  SalesDetails();

  SalesDetails.fromMap(Map<String, dynamic> data) {
    id = data['id'];
    purchaseItems = PurchasedItems.fromMap(data['purchased_items']);
    bankTransferSenderName = data['bank_transfer_sender_name'];
    chequeInfo = data['cheque_info'];
    creditPayLaterInfo = data['credit_pay_later_info'];
    customerId = data['customer_id'];
    customerFirstName = data['customer_first_name'];
    customerLastName = data['customer_last_name'];
    subTotalAmount = data['sub_total_amount'];
    totalAmount = data['total_amount'];
    taxesDue = data['taxes_due'];
    isComplete = data['is_complete'];
    createdAt = data['created_at'];
    updatedAt = data['updated_at'];
  }

  Map<String, dynamic> toMap() {
    return {
      'id': id,
      'purchased_items': purchaseItems,
      'bank_transfer_sender_name': bankTransferSenderName,
      'cheque_info': chequeInfo,
      'credit_pay_later_info': creditPayLaterInfo,
      'customer_id': customerId,
      'customer_first_name': customerFirstName,
      'customer_last_name': customerLastName,
      'sub_total_amount': subTotalAmount,
      'total_amount': totalAmount,
      'taxes_due': taxesDue,
      'is_complete': isComplete,
      'created_at': createdAt,
      'updated_at': updatedAt,
    };
  }
}

//PurchasedItem model

class PurchasedItems {
  String itemId;
  String itemName;
  String itemDescription;
  int itemQuantity;

  PurchasedItems();
    
  PurchasedItems.fromMap(Map<String, dynamic> data) {
    itemId = data['item_id'];
    itemName = data['item_name'];
    itemDescription = data['item_description'];
    itemQuantity = data['itemQuantity'];
  }

  Map<String, dynamic> toMap() {
    return {
      'id': itemId,
      'user_id': itemName,
      'item_quantity': itemQuantity,
      'item_description': itemDescription,
    };
  }
}

After watching the official Firestore videos on Maps vs Arrays vs Collection/documents, I've decided that sub collections / documents might just be the best for me due to how I am to display the information on my app.

1. my toMap and fromMap methods, are they rightly implemented?
2. Is this nested too deep?
3. Should I leave Sales details and purchase items as maps or are they fine as their own classes?
4. Is there a better way to my current implementation? What's the recommended standard of working with models who depends on other class models?

Thanks.