Just another way to look at this: Is it the frame, or is it the wheels?

Could it be the frame is flexing reasonably under that weight, but your wheels are not? I can't say I cycle with that much gear, but have had wheels in the past (usually machine built) that have flexed too much until I've rebuilt the wheel. Or perhaps with that kind of weight are 48 spokes in order? I'd think with that much weight you might need some really well built wheels -possibly with more than the traditional 36 spokes.

It might be worth it to beef up your wheels to see if it makes a difference, and then swap them over to the LHT if you decide to get one. Could be an easier and cheaper way to work out if the 520 is the problem.