12 speed Di2 rim brake wired only. Why?

(Automotive) brake cleaner in a spray can to clean the rotor and pads with a paper towel has always worked well for me. And consider some sandpaper to resurface the pads. Be sure to re-bed the brakes afterwards. I have found organic pads tend to be much quieter in all circumstances vs sintered-metal pads, but organic pads don’t last nearly as long. I’ve had a lot less problems with brake contamination since I started waxing my chains, but that’s a whole other topic.