Unfortunately it's not as if we're talking about Cadillacs vs. Lincolns here. There are no suspension systems on lawn tractors.
There are only about three factors involved with ride comfort of a lawn tractor; tires and pressure thereof, machine weight vs. driver weight and possibly seat configuration.
Bigger tires with less pressure will yield a smoother ride. Try lowering your tire pressure as low as you can get away with.
A heavier machine with a lighter driver will feel better than a heavier driver on a light machine.
In some cases a better seat with a taller back will help, but more for general comfort than for soaking up bumps. For this purpose it would be nice to adapt a sprung tractor seat to your mower. Even my ancient old Ford 3000 has a comfortable seat with a full back and an adjustable spring ride mechanism. I don't think it is the original seat for that tractor, but it sure gets the job done. A trip through a tractor salvage yard might turn up something that could be adapted and GREATLY aid in comfort.
Good luck,
Doc