I know exactly what you are going through!
My B2410 is exactly the same on rough ground, and my ground gets rougher every year. I am not sure if you have the treadle pedal that you rest the foot on, or the under deck treadle, which is what my B has, but I found a way to rest my foot so the bouncing doesn't affect the pedal. If you can, replace the seat with a suspension seat, your butt and back will thank you, and will make it less likely to jounce the treadle.
The MX has the same "type" HST as my B does, direct connection to the valve, so not much you can do to adjust sensitivity, it's just something that trial and error and time get over. I have an L4060 HSTC on order which is electronic controlled and probably less sensitive to bouncing, I hope. It also has a suspension seat, which will make it much easier on my back. Right now I have a 4' rough cutter on my B2410, and the L4060 would have no problem with a 6' deck on it.
With the market today, you might be able to sell your MX and get an L4060 or L6060 (IF you can find one) and not lose anything in the process, unlike a couple years ago. I think you might like it better than the traditional one you have now. Nothing wrong with the MX, but it is the Chevy version where the Grand L is the Cadilac version.