I would say all you need to do is take a picture of the sky on a cloudy day, then at a pretty good distance from the window in this scene, add a flat plate then put the picture of the sky you took, and set shaders on it to FLAT. Very easy on your hardware and time, and looks good. Also perhaps the lighting could use a little warming up. Some slight yellowing or blue in it should help. I don’t know, just my opinion. And you said you have several lights? Try only having the light coming in through the window, and one in the bathroom to help it glow, then turn up the intensity of the lights. But if that doesn’t help then go ahead and keep it.