You might consider looking at the R+M Load or Packster instead of doing the trailer. I don't know if your stuff will fit but you won't have to tow a trailer and while yes you would still be braking downhill rather than locking out speed, the bike is well designed to handle heavier loads of kids and cargo. It take a little getting used to but the Load especially is a fantastic bike and rides really well. Potentially your local dealer (which may not be super local) could install bigger rotors (and all the needed adaptors) on the system but the stock is pretty good especially with the 4 piston brakes. I would go with an RT86 IceTech rotor in 203mm at the rear and see if you can put a 180 at the front if you are worried about it. Plus the new 2020 stuff will have the new Bosch Cargo specific motors so you can get better torque and a motor designed for handling heavier loads.

Yes the R+M stuff is more expensive but it is some of the finest cargo carrying bikes that exist.