If your wheel failures are from breaking spokes, not bending or cracking rims, it sounds like the wheels were originally built with inadequate spoke tension.
Spokes break from fatigue, not from excessive load. An excessively tight spoke will pull through the rim bed way before it will break from tension. However, a too loose spoke will fail from fatigue since it goes too slack on every rotation and the fatigue loading is increased.
Have your wheels rebuilt by a competent builder who will get the spoke tension properly high. Also, as counterintuitive as it seems, butted spokes are more durable than straight gauge as the thinner center section can flex more and reduce the shock loads on the thicker ends. Remember, spokes always fail at the ends, not in the middle. (Unless you hit something