350 pounds of riders - plus the weight of the bike - with a 4 year old/20000 mile rim brake carbon front wheel.... Glad for you that this didn't end badly and I must say I admire your sense of adventure!
I think that you were on the right track by using a disc on the rear. But, the physics is such that the rear brake does not provide nearly as much braking power as the front, and a hydraulic disc brake is going to give you much more braking power than the mechanical disc you have.
I suggest that if you are going to ride carbon rims that you get yourself a nice set of hydraulic discs - either full hydro from the lever or cable-actuated hydro. And, make sure your front fork can handle the load and disc braking forces you are intending to apply.
Good luck!