Ah interesting, for my MX 3 the thumb button doesn't show as button 5 but instead it acts as cmd+tab.
Independent mouse gestures are indeed not easy to achieve at the moment, but it can be done. I'll post an example tomorrow (currently don't have my mouse with me to test).
Maybe the floating menus as shown in the example here
or here:
could also be an alternative to mouse gestures (they can be made to be shown on mouse down, and trigger the hovered action on button release)
