Failure at the rim happens sometimes, but that seems like a lot. This is sometimes the result of excessive(or uneven so that some spokes are excessive while average is not) spoke tension, sometimes the result of poor materials or manufacturing of the rim, and occasionally can relate to particularly high torque loads placed on the wheel. Still, with your history, I'd recommend getting some sturdy, conventionally designed wheels hand built by a reputable builder. Well built wheels should not loose much if any tension during use because they will be built at high, even tension and will have the spoke line set during the building process and will be properly stress relieved, and will also have some mechanism of thread locking on the nipples to keep them from unthreading if and when the spoke is detensioned during riding.