I believe the solution shown in your screenshot will not work if there are more than the two topography’s in the project. Because that graph is only removing the topography that is hosting the subregion, correct?
Also, the SiteSubRegion does not inherit Revit.DB.Element but rather .NET Framework System Object.
Please correct me if any of my statements are incorrect.
So, does anyone know of another way to convert SiteSubRegion object to Dynamo Element?
I have opened a new thread regarding this topic here.