Don't overlook a critical element that dictates ride characteristics
I've ridden a lot of different ztr's and obviously the ones with a suspension system of some sort are softer riding than those without. But, one of the easiest and cheapest (free in fact) ways to soften the ride of any ztr is to lower the air pressure in all 4 tires to 7-10 psi (depending on what type tire you have).
Considering how rough the ride is on a ztr (especially one without any suspension of any kind) and considering how many people like myself have back problems, I'm amazed at how often something as critical as air pressure is overlooked. A ztr's small diameter wheels/tires make the factor of air pressure extremely critical when it comes to ride quality....so starting there when looking to improve ride quality is what I'd suggest.
Even with things being at their best, in most cases the ride is still going to be on the rough side because of the roughness of most lawns and the small diameter of the ztr's wheels/tires. Being small diameter means they're going to go down into every little rough spot compared to the large diameter of a tractor wheel/tire, which simply rides over small rough places as if they weren't there.
Shut 'er down Buford....she's slangin' oil !!!!