I had to board an airplane so I couldn't type a full reply earlier. Diffraction limits are different based on the sensor size, pixel pitch, and the lens optics, and diffraction affects sharpness even with a more open aperture, it's just limited in comparison to the impact of increasing depth of field as you stop down. Part of composing a scene is choosing how you want to balance DOF / sharpness, which can go in many different directions depending on what you're trying to achieve.

It's simply not the case to say that diffraction doesn't affect sharpness below f/11, and diffraction is not the only impact that can affect outcomes from stopping down, when you stop down you are letting in less light over the same sensor area which affects almost every aspect of exposure, and has to be compensated for either by increasing ISO which increases noise or by reducing shutter speed which limits motion compensation when shooting handheld, all of which can affect the level of detail that is rendered sharply in a frame, either due to blurring or due to unrecoverable noise.

Generally, my personal preference is to stop down enough to get a sharp frame edge to edge across the center when trying to capture wide scenes, and no more, on many lenses f/4 is enough, generally no more than f/6.3 is required. You begin making serious tradeoffs as you stop down further, especially if, like me, you shoot handheld almost always, and often manually focus (e.g. subtle movements can affect your critical focus distance).

Your rule of thumb is largely irrelevant, you should be making these decisions each time you make an exposure to achieve whatever artistic effect you are going for.