My shed is from Postwoodworking of New Hampshire.
It's 4x4's pressure treated skids like yours on pavers at 3/4" crush stone grade (stone under entire shed), and then 2x4's pressure treated floor joists like you but I believe they are 16" on center and 3/4" genuine plywood (not OSB) for decking.
Some areas feel a little soft w/ the tractor, but it's been OK. They gave me a weight rating and I can't remember what it was, but obviously it can handle the ~3000 lb tractor (mower deck and loaded tires get it up there).
I have turfs, so they distribute the load nicely especially the rear of the machine.
I would think w/ 12" apart 2x4 joists and R4's, ag, or turf you'd be fine.
I've been thinking about ripping up the area where the tractor drives over to beef things a bit or possibly even just add another layer of plywood for some insurance. I worry more about the front tires as they have the smallest foot print between two "joists". The rear tires are always supported by 3 or 4 joists across the two tires. So I keep the bucket down at floor level at all times just in case a front tire would hit a potential soft spot just right - I could "back blade" my way backwards and then fix any floor issues
I've had 2 seasons of no issues, so I think I'm set.
I support the center of my ramp w/ paver brick and the section that meets the edge of the shed w/ a 2x6 pressure treated acting as cleat w/ paver bricks in case the screws holding 2x6 to shed every fail -- pavers support the 2x6 "i-beam". So the ramp is on the ground, pavers in the middle, and paver supported 2x6 cleat to side of shed -- all made out of pressure treated. It's solid as a rock.
If I think of it I'll take some pictures of my CK-20's home inside and out -- very comfortable for it.
Winter time she lives in the garage w/ one car left outside (at least when a storm is approaching) so if we get a huge snow storm I don't have to dig my way down from the shed to the driveway -- just plow my way out of the garage.
Enjoy.