Can’t you just handle the GameWindow.ClientSizeChanged event:
this.Windows.ClientSizeChanged += Window_ClientSizeChanged;
and in the handling code react to the new window dimensions?
void Window_ClientSizeChanged(object sender, EventArgs e)
{
int width = this.Window.ClientBounds.Width;
int height = this.Windows.ClientBounds.Height;
//do something with the new width x height
}
As for setting isfullscreen = true, it has no effect in Windows 10 (Monogame v3.4).