How do I show a MessaegBox after my main Window and it’s contents are all loaded and visible?

Use the Dispatcher object to make your method run after all the ‘background’ operations are completed, as follows:

public Window1()
        {
            InitializeComponent();
            this.Dispatcher.BeginInvoke(DispatcherPriority.Background, new SomeMethodDelegate(DoSomething));
        }

        private delegate void SomeMethodDelegate();

        private void DoSomething()
        {
            MessageBox.Show('Everything is loaded.');
        }