CSF - Multibutton mouse solution
I use the same optical mouse between my PC and Mac, and have always been annoyed that the "forward page / back page" buttons on the side didn't work in MacOS. There didn't seem to be any way to map them natively for that behavior. Then I came across this https://sensible-side-buttons.archagon.net which mimics the three-finger trackpad swipe to make use of those buttons. It seems to work across the OS, not just in Safari. Hats off to the developer.
I've just been using BetterTouchTool for that kind of thing.
I want to use it too.