I use SteerMouse. It lets you customize mouse speed/acceleration, remap all your buttons, and set up mouse chords. It's admittedly paid and it's stupid that yo

I use SteerMouse. It lets you customize mouse speed/acceleration, remap all your buttons, and set up mouse chords. It's admittedly paid and it's stupid that you should need to pay for functionality like this, but it's useful enough for other stuff (e.g. setting up buttons/chords to manage spaces to replace the trackpad gestures) that I live with it.