Rear wheels are indeed subject to much more weight and force, no matter how heavy and mighty you and your guads are or are not. As for 12k being middle-aged for a wheel, it all depends - they're cheap, machine-built wheels subject to much outdoor riding, and do I recall that you're not just a fair-weather cyclist? I'm not at all surprised you're running into issues, and the next spoke breaking during retensioning is to be expected, since they're all the same age and subject to the same conditions. One of AG's old bosses/ friends had the exact same sort of issue (only he's heavier than me), and his spoke lived long enough to break a month later and ruin a second ride. When a spoke breaks, or is compromised, I'd go with a complete rebuild. Or, if you feel like splurging, hand-builts from November or similar with nicer rims and hubs than Spesh bundled.