I would say a larger and heavier garden tractor would be more comfortable than your light weight lawn tractor. But maybe the cheapest and best approach would be to install an aftermarket suspension seat on your Sabre. (see link below)

Here is a link that might be useful: suspension seat

I agree with the point made by larso.
If you are completely satisfied with your Sabre, buy a quality suspension seat.
Keep your current seat stored for reuse if or when you decide to trade up to a different tractor.
The suspension seat can then be used on the new tractor and your Sabre can be sold/traded with the OEM seat.
Another point to be mindful of regarding "quality of ride" is that you do not overinflate the tires. While tire pressure is usually discussed in terms of people keeping up with having ENOUGH pressure in their tires...........there are the few who simply pump 'em up till they feel "right".
That can easily result in too much pressure and a subsequent hard ride.

and to accomplish that usually means investing a few dollars in a better quality air pressure gauge that will accurately register in the 10 - 20 PSI range. Most tire gauges are designed to work best with pressures of 25 - 50 PSI and often will not yield accurate repeatable readings at lower pressures.
Not only getting your tire PSI lowered for ride comfort, but having both rear tires matched in pressure will help in maintaining that level cut...

OTOH, low PSI can easily lead to flat tires caused by the tire's rim bead becoming separated from the rim proper during normal flexing - usually happens when PSI is 10 or less...

Next spring - when the ground is still soft and if your Sabre is capable - rent & pull a roller across your cutting area to help smooth out those humps/bumps... and then slow down a bit when cutting. Do believe yer back will thank you (got several of them t-shirts) :)

Thanks for all the suggestions, guys! While I can't pony up the $390 for a suspension seat system like I saw in the link, I could spend $125 or so for another seat. I checked and found one that is compatible with my Sabre, but while it will fit, it still uses the existing little springs on the tractor body, but the seat has a higher back. Will that be more comfortable than the low backed seat that is stock?

Probably not much of an improvement. My wife (RN) says get a pillow wedge and take some pain meds. Not much else I could subscribe for less $.