Your observations and previous responses about diffraction are correct, however you might be exaggerating it's effect in your application. It's something you have to consider when using digital cameras. Diffraction "robs" fine detail from an image and at it's worst can make an image look soft and "plasticky" overall, similar to running really strong noise reduction on it. There's a calculator at that link to let you determine the Diffraction Limited Aperture of any camera.
However, are you using a 5D classic?
The reason I ask is that diffraction is effected by three things: the density of pixel sites on the sensor, the size of the lens aperture, and the final use/output of the image. Full frame cameras have far less crowded sensors than crop sensor cameras, so are more resistant to diffraction. A specific example... the 21MP 5DII has less than half the pixel sites per square millimeter, vs the 18MP crop cameras.
If you calculate the Diffraction Limited Aperture (DLA) for an 8x10" print from a 5DII you'll find it's f10. For a 7D or 60D or T3i or T2i, the DLA is f7.1.
Now, the DLA is where diffraction first starts to set in and you probably won't be able to see it with your naked eye. In fact, it increases gradually from there and you can probably use a bit smaller apertures without much concern. For example, I'll use f8 and sometimes even f11 with my 7Ds. With 5DII, I'll use f11, f16 without worry, sometimes even f22.
If you are using a 13MP 5D classic, with an 8x10" print it has a DLA of f15. You can almost certainly use f16 and probably f22 without much concern.
Read the details about diffraction at the above website... As it notes, it's a trade-off, like everything in photography. A little diffraction might be just fine, if allowing it to occur offers greater depth of field that's needed for a successful image. Also, note that the viewing distance with the final product is important. Often when calculating DLA an 8x10 viewed from a normal distance is used. If instead a 24x36" print were made, viewing distance would change, otherwise one would be overly critical of the image. In other words, viewing digital images on a computer monitor at 100% might be a bit like walking up to an old master's painting in the Louvre until it's a few inches from your nose... what was a masterpiece from a normal viewing distance is nothing but a bunch of messy brush strokes from a few inches away.
Don't confuse diffraction with the optical limitations of lenses. Large aperture lenses are designed to work best at their larger apertures, of course. They might not be at their best stopped down. So a lens that's designed to work best at smaller apertures, such as a macro lens, might give you better results.
You probably are aware, DOF is controlled by aperture size, focal length and working distance. So if you are looking for more DOF, you might try backing off a little, and/or using a slightly shorter focal length, then shooting the image a little loosely to allow it to be cropped later. A limitation might be the size of your studio, how much you are able to back up. Ever notice what fashion and large product photographers tend to do? They use quite large studios where they can shoot from quite a distance with a moderate telephoto lens. 200mm lenses are popular portrait lenses with fashion photographers, while most other portrait shooters traditionally use 85mm and 135mm. The longer lens natural renders shallower DOF if used at the same working distance, however they have plenty of room to allow them to back up and shoot less tightly, making for more DOF with the same aperture. (Another reason fashion photogs use longer lenses is the subtle perspective flattening effect that occurs.)
You might want get a good test target, something with shape and lots of texture and fine detail (maybe a large, furry, stuffed animal?), to run a series of tests with your lenses in your work space.... to find the optimal apertures and working distances of your lenses on your particular camera. You might need to make some prints initially, since something is always lost viewing images on a computer monitor, but then can probably do most of the analysis comparinig images on your computer. Boring stuff, but a test series can help you develop a plan for how best to use each of your lenses, what their limits are. Without shooting a boring test series, it can take years to learn the nuances of gear performance and how to optimize it. The tests can be a short cut to getting the best out of a particular piece of gear (testing done every so often also might find gear that needs repair or calibration).
The only other things I can think of... Have you had the focus calibrated on your camera lately or run Micro Adjust on each of your lenses, if the camera is a model that has that feature? And, do you have protection filters on your lenses? If so, try shoothing without them. Smaller apertures can make filters have greater effect on images, particularly with wider lenses.