The Aksium will hold up, but take into consideration that it's a lower spoke count set, so if anything goes wrong with one spoke, it will be of greater consequence.

If you want a rim that stays close to the 20mm side profile you have now, check out one of the standards of this forum; 32h Mavic Open Pro rims on Ultegra hubs. They're not really a bank breaker at $150 for the rear and $130 for the front. A quick stress and tension (since they're usually stock machine built sets at places like Performance Bike), and they're good to go. I've got almost 1000 miles on an Open Pro/Ultegra rear from Performance right now, and I weigh 250 also.