Xamarin SDK for OpenStreetMap

I'm trying to make a Proof of Concept (C#) for basic graphical maps, as an alternative to Google Maps for Andriod and iOS devices - because Google started charging fees for their APIs (from my understanding only affecting web right now).

I doesn't need to be particularly advanced, simply a GUI that shows a base map where you can draw:

• Markers
• Lines
• Polygons

The only requirements I have is that it should be open-source, or at as low a cost as possible.

What I've done so far is to use data from http://openstreetmap.org - and set up a tile-server https://switch2osm.org/serving-tiles/ on a separate linux machine.

Furthermore, it went fairly quick to create a simple web app with OpenLayers.js and Leaflet.js connected to the custom tile-server with the requirments met.

What I need to do now is to find a free or cheap mobile SDK for Xamarin for Android and iOS. I managed to render a map from my own tile-server and add markers by referring .dll's from this zip from 2014 (only tested for Andriod): https://github.com/OsmSharp/ui/releases/tag/v4.2.0.723

 using OsmSharp.Android.UI;
 using OsmSharp.Android.UI.Data.SQLite;
 using OsmSharp.Math.Geo;
 using OsmSharp.UI.Map;
 using OsmSharp.UI.Map.Layers;

 [Activity(Label = "@string/app_name", MainLauncher = true)]
 public class MainActivity : AppCompatActivity 
 {

     private MapView _mapView { get; set; }
     private Layer _mapLayer { get; set; }  

     protected override void OnCreate(Bundle savedInstanceState)
     {
         base.OnCreate(savedInstanceState);

         try
         {
            Native.Initialize();

            _mapView = new MapView(this, new MapViewSurface(this))
            {
                MapTilt = 0,
                MapCenter = new GeoCoordinate(lat, long),
                MapZoom = 16,
                Map = new Map()
            };

            // create a marker under Resources/drawable/pin.png
            using (var bitmap = BitmapFactory.DecodeResource(Resources, Resource.Drawable.pin))
            {
                var marker = new MapMarker(this, new GeoCoordinate(lat, long), MapMarkerAlignmentType.CenterBottom, bitmap);
                _mapView.AddMarker(marker);        
            }

            _mapLayer = _mapView.Map.AddLayerTile("http://*.*.*.*/{0}/{1}/{2}.png");
            SetContentView(_mapView);
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex);
        }
    }        
}

However, these .dlls seemed to lack support for Lines and Polygons. I have tried to get something similar to work with OsmSharp's latest NuGet package (2018-06-04), but my novice Xamarin experiance only gets me so far.

Does anyone have any tips on how to use my own tile-server and render native maps on Android and iOS devices?

PS. It doen't strictly need to be OpenStreetMap with OsmSharp connected to a custom tile-server, that's just something im leaning towards right now. Again, requirents are open-source or at a low cost with the fexebility to add Markers, Lines and Polygons.