Really, the first questions should be, "Do I need diffusion?" and "Why?" Diffusion is a good addition to most listening rooms, I'd wager. But, as you suggest, "where" is the hard part. Diffusion acts to redirect and delay reflected energy with minimal energy loss, compared to plain absorption. So, without knowing anything about the energy decay in your room we can't say with much confidence. Still, we can make some generalizations, I suppose (though I am not by any means the most qualified to do so).
Diffusion of some sort is often recommended for first reflection points on side walls and ceilings. If the room size, speaker location, and seating position lead to a very high gain set of early reflections it might be most appropriate to absorb or pursue a hybrid design that both absorbs and diffuses. Or maybe 2D diffusion reduces the gain enough to maintain the purity of the direct speaker signal (if that is what you want) by redirecting and delaying the energy's arrival at your listening position. Or you might find that 1D diffusion does a better job of letting you enjoy the wide soundstage that your speakers might create. See how there's lots of options? The choice hinges on several things: room size; speaker location; seating location; speaker dispersion/off-axis performance; content (stereo vs multi-channel music vs home theater); and last-but-perhaps-most-important - preference. (Have I confused this enough? Go back to the first sentence of this paragraph.)
The details of all those parameters (especially speaker design and location details) would suggest the bandwidth you might need for diffusion. Once that is known, you can start to compromise and fail to meet that need, most likely (nice, huh?).
The bandwidth of most diffusors is defined by the depth of the deepest wells (the difference between the tallest and shortest sections) and the width of the wells (the 3x3 you indicated in your design). In order to extend diffusion to lower frequencies the wells should be made deeper. In order to extend the high frequency limit upward, the wells should be narrower. Once those two features are defined, you need to determine the pattern that fits in your space and that you can build - it's not random.
The whole process can be a little daunting and gets pretty complex. Here's a link with some good background reading links that you maybe haven't seen: https://www.gearslutz.com/board/bass...-diffusor.html
As a practical matter, it might be simplest to combine your needs and desires and basic understanding with the compromises you can identify in popular commercial products, and emulate them.