There's a couple things you're dealing with here, pumasong. The first is finding matched turf tires, and the other is getting wheels to fit whatever tires are compatible. The calculation you did for your circumferences isn't correct if the diameters are correctly measured (I think you doubled the diameter), so I found this tire website:
https://www.bkt-tires.com/ww/en/tr-171 that lists the rolling circumferences for the 6.00-12 fronts (1953mm), and 8.3-22 rears (2778mm). When I compare the ratio of front/rear, I get 0.703.
As long as you find tires that are near that proportion, you should be safe. Carlisle lists actual mounted diameters for most of its products.
Multi-Trac C/S Tire
The width of rim they are mounted on as well as the pressure will affect all of the measurements, of course, but it should get into the ballpark at least for safe operation.
It also seems like there is a reasonable assortment of turf-pattern tires in various heights that should fit your 12" front rims, so the issue will be figuring out what to do about the situation out back. As best I can tell from your measurements and pictures online, the bolt pattern is 6x120mm.
It looks like some GM crossover-type SUVs use the 6x120mm bolt pattern, though with a different hub bore. The wheels seem to be 18" or 20" diameter, which doesn't seem to help with turf tire availability. However, it does mean that there are some commercially available adapters. These seem reasonably priced for the rear
2X 1.5" Wheel Adapters 6x12 to 6x139.7 6x5.5" Fits Chevy Canyon 6Lug Conversion | eBay
The 6x5.5 would allow you to use Toyota, Nissan, and GM 1/2 ton 4x4 wheels, along with many agricultural implement wheels. These are all commonly available with 15" and 16" wheel diameters. This is handy, because those wheel diameters would allow the use of the most common turf tires out back. You will need to find a rim 10" wide, probably, at a bare minimum, which likely means finding aftermarket 4x4 type. Otherwise, you will probably need to use tractor wheels, which may not match a conveniently available adapter like the automotive wheels would. You may also have to deal with backspacing issues with automotive wheels, though the adapters will help with that.
All that said, I would suggest that the turf tires and wheels you linked on ebay are a great option. The rear tires are the most expensive item, and you can have an adapter made or modified if one isn't available to match the 2520's bolt pattern.
Most of the time on grass you should be running in 2 wheel drive anyway, so I wouldn't have excess concern about the size mismatch. By my numbers it would be better if the fronts were about an inch taller, but I think you might decide to buy front tires that mount on your current wheels anyway. The spacers are fine out back, but offsetting the front wheels with a spacer may adversely affect the steering geometry. I would try to avoid spacers or adapters in front if possible, so if the bolt patterns don't match for the wheels off the 2520, I would put different tires on your current front rims before adapting the others. This is an interesting project, I wish you luck, and look forward to seeing pictures!
