You mainly have to flag off direct sunlight so that you only have cool skylight coming into the room. Second is to avoid a background view that has full sun on it. You could try ND gelling that window with a sunny background but just remember that you will also kill the amount of soft skylight coming into the room from that window, but that might be OK.
Trouble with putting diffusion on a window that has direct sun is that the sun will light up that diffusion and make it very bright and hot to camera, so it only works for an off-camera window where you are just trying to soften the light. Since it is off-camera, almost any diffusion will work, but with full sun coming in, something heavier might be better and softer, something like 216 or 1000H or Full Grid. You may also want to put 1/2 CTB on that diffusion to cool the color off because diffused sunlight is warmer than skylight. Unless you want some warmer soft light coming in as if an off-camera sunset sky.
If the sunny window is in the shot, it's better to flag the sun from above the window outside, like a shelf. Or diffuse every window, cover all of them with blinds or sheers to break up the whiteness, and play things in silhouette in the room against these whited-out windows. Would look moody but generally at dusk you'd expect to see detail outside the windows, not even whiteness.