Have you considered an upgrade to the 1DX mk.II? It ticks all your points, except maybe quiet shutter: it is quieter than normal, but not as silent as an electronic shutter which will lower your dynamic range anyway so it may not be worth the trouble for you.
You already know the system and have the lenses, so the upgrade will be quite seamless. 20MP are plenty for advertisement as long as your pictures are sharp. You could go for a Nikon D810 or Sony a7RII for more pixels and a slight dynamic range advantage, but you will be sacrificing the AF performance and switching to a new system which may give you a hard time getting used to.