The only thing lower than that bike is bigbox stuff. So you can't expect much. Even less with a big rider.
If 2.3 bend DS spokes don't help, then you need a better bike.
My Sturmey Archer heavy drum brake IGH hubs all have 2.3/ 2.0 spokes. Impossible to break.
My Rohloff14 tour bike is 120 lbs loaded. Nothing broke but the first skinny fork.