Anytime I've ever had to map extra mouse buttons I've always had to do a discovery step to figure out what "button number" any specific mouse button mapped to.
Is that for back/forward, or extra buttons on top of those? Back/forward have pretty standard meanings & usages so I'd expect them to work out of the box (and they have for me). Can't comment much on other extra buttons though, quite possible those don't really have a good way of being handled.
Anytime I've ever had to map extra mouse buttons I've always had to do a discovery step to figure out what "button number" any specific mouse button mapped to.