There isn't a great way to serialize this information. If you want to serialize the data, you should be using Features, not Graphics. Graphics are designed for temporary display. This help doc explains this in detail - Features and graphics—ArcGIS Runtime SDK for Qt | ArcGIS for Developers
Instead of Graphics, I'd suggest you use FeatureCollectionLayer. FeatureCollectionLayer is the view and the data comes from a FeatureCollection. The nice thing about this class is that it implements JsonSerializable, so once you create a FeatureCollection, you can call toJson, and send the JSON to someone else, then call fromJson and create a new FeatureCollection and give it to a FeatureCollectionLayer - FeatureCollection Class | ArcGIS for Developers
