You could also use rim brakes in addition to the disc for a bit of load spreading...but you don't want to keep running those hard because if you heat the rims enough the air in the tires may expand enough to exceed the pressure they can take.
How do you use 200mm rotors with TRP Spykes if all large rotors are for narrow type pads and TRP Spykes are using wide type pads? This creates accelerated pad wear according to Shimano.Tires getting heated up is a real problem. We have bike toured in Italy in the Dolomite mtns many times and had tubes over heat and burst. Disc brakes were the worst brakes by far. Rim V brakes with rest stops to let them cool worked and my wifes bike we had rim brakes and a drum.
The problem with mechanical brakes on the trailer is running the cables and having them tether all the time. I need something like the car trailer brakes that actuate when there is pressure on the hitch. With hub motor brakes I could rig up a wireless actuation from the bars.
I guess I need to see how the bikes brakes work alone and then decide. I am usiing TRP Spykes with 200mm rotors. My previous experience says it won't be enough.
Thanks for the thoughts everyone.