I should have asked earlier how are you creating the symbols for the graphics. I believe you are using multilayer symbol. If not, then I recommend using Multilayer symbol for the graphic using the example provided below.
If you are already using Multilayer symbol then choosing the right color for the bottommost stroke symbol layer and not having to apply any additional transparency should work for your use case. If you need additional transparency you can set it GraphicOverlay.
overlay.Opacity = 0.8;
Below are two images I captured with code snippet below. First one is GraphicsOverlay with 0.8 opacity and second one is without opacity.
var overlay = new GraphicsOverlay();
polylineBuilder = new PolylineBuilder(SpatialReferences.Wgs84);
polylineBuilder.AddPoint(new MapPoint(-30, 20, SpatialReferences.Wgs84));
polylineBuilder.AddPoint(new MapPoint(30, 20, SpatialReferences.Wgs84));
polylineBuilder.AddPoint(new MapPoint(-30, -20, SpatialReferences.Wgs84));
polylineBuilder.AddPoint(new MapPoint(30, -20, SpatialReferences.Wgs84));
var tranpstrokeLayer = new SolidStrokeSymbolLayer(20, Color.Gray);
tranpstrokeLayer.CapStyle = StrokeSymbolLayerCapStyle.Round;
strokeLayer = new SolidStrokeSymbolLayer(3, Color.White);
strokeLayer.CapStyle = StrokeSymbolLayerCapStyle.Round;
dashEffect = new DashGeometricEffect();
dashEffect.DashTemplate.Add(7);
dashEffect.DashTemplate.Add(9);
dashEffect.DashTemplate.Add(0.5);
dashEffect.DashTemplate.Add(9);
strokeLayer.GeometricEffects.Add(dashEffect);
lineSymbol = new MultilayerPolylineSymbol(new List<SymbolLayer> { tranpstrokeLayer,strokeLayer });
var dashDotGraphic = new Graphic(polylineBuilder.ToGeometry(), lineSymbol);
overlay.Graphics.Add(dashDotGraphic);
Few resources: https://github.com/Esri/arcgis-maps-sdk-dotnet-samples/tree/main/src/WPF/WPF.Viewer/Samples/Symbolog...
https://developers.arcgis.com/net/api-reference/api/net/Esri.ArcGISRuntime/Esri.ArcGISRuntime.Symbol...
https://developers.arcgis.com/net/styles-and-data-visualization/symbols-renderers-and-styles/
Hope this helps.