Sorry to hear about your wheel troubles!
On the subject of Mavic Ksyrium wheels for heavier riders, I'll quote what a Mavic representative had to say on the subject here:
We generally suggest Ksyrium Elite for heavier riders doing road or Cyclocross. But depending on how hard the guys ride, even our strongest wheel systems will have a finite lifespan. The systems wheels like Ksyrium and R-Sys are built to accommodate the middle of the bell curve when it comes to rider weight and style. 250-pound riders for sure fall to the extreme end of the bell curve.
Therefore, quite honestly most heavier (250 lbs+) riders will be best served with custom built wheelsets.
Now, onto the subject of a quality custom built wheel, here is what I can say:
1) The DT Swiss TK 540 rim is one of the laterally stiffest rims on the market. It was designed for loaded touring, and in a clyde application it should have no problem staying true and not breaking spokes for even a ~300 lb clyde. My new back wheel is a 36h TK 540 laced to a Shimano Ultegra hub with DT Swiss Comp spokes, brass nipples and spoke washers. I've only got about 200km on mine but it has stayed dead true so far where my previous 36h Mavic Open Pro would come out of true after a single 30+km ride (despite repeated truings). The 23mm rim width also has benefits vs. the 19mm Velocity Deep-V that is often recommended for clydes (the TK 540 is also lighter).
2) No matter the quality of the components, a custom built wheel will only be as good as the wheelbuilder. Don't learn the lesson I learned the hard way with the piece of garbage 36h Mavic Open Pro I had built by my clueless LBS. Find a wheelbuilder who actually has useful advice for a clyde (i.e. has experience building wheels for clydes), has and uses an tensionometer to check and balance spoke tension, who stress-relieves the spokes before delivering the wheel to you, and who is willing to offer a long-term warranty on truings and non-impact spoke breakage.
3) Worry far more about back-wheels than front. I'm ~285lbs and on my new bike I didn't even bother to switch the Giant P-SL0 front wheel, notwithstanding its low spoke count, because I've never had a problem with any of the front wheels on any of my bikes. In my experience, high spoke counts on front wheels are generally not necessary, even for clydes. Perhaps others on this forum can chime in if they've had different experiences.