Take a comprehensive approach
Lot’s of good advice already given in this thread. Here is my two cents: The real issue is that your are a bigger, heavier, and probably stronger person than the average rider. The loads on your rear wheel are enormous. Therefore, a relatively standard build like you describe (Mavic open pro 32 hole, 3 cross, DT straight gauge) is probably never going to be durable enough for you, no matter how expertly built. My advice as follows:
1. 36 holes, 3 cross, both front and rear.
2. Stiffer rims, some sort of deep V is probably called for. Do not go for weight savings, go for strength.
3. Do everything you can to equalize the tension on the drive and non-drive sides of the rear. The easiest way to do this is to get a hub that has a larger drive side flange than the non drive side. This works to equalize (between the two sides of the wheel) the angle at which the spokes leave the rim and effectively reduces the need to have the drive side at a greater tension. This in turn allows, you to increase the tension on the non-drive side and still maintain proper dish.
Bottom line, the real fix is probably a new set of wheels. I am sure the members of the board that are more expert than myself could recommend a parts list for the build strategy outlined above.
I realize that this is not the cheapest fix recommended but it does address the fundamental issues. If you do go this route, you can rebuild your current wheels and have them as a back-up set.