I have a Logitech G305 wireless mouse that I use with my computer. I want to be able to assign “Show desktop” to “Mouse button 5” (top side button on the mouse), but it works very unreliably. A couple of times this has worked when I set it up, but it quickly stops working for some reason.
There are others who’ve had the same problem (Activating Show Desktop in Mission Control with a mouse button does not work), where one solution was to download Logitech’s software and map it to Show desktop from there. The G305 does not yet have software for macOS, unfortunately.
There is nothing wrong with the button itself either. If I assign Mission Control to Mouse button 5, it works as expected. Show desktop? Not so much. It also functions as “Open in new tab” in Safari, and I imagine it has other uses in other applications.