The "green" antifreeze is a whole lot safer than regular AF or windshield washer fluid if you spill or have a puncture. RimGuard is made from beet juice, but I don't know what else is in it.
An alternative that may be totally unworkable is sugar dissolved in the water. I've never heard of it, but I see no reason why it wouldn't work in your situation. It's not corrosive like the various salts, is environmentally safe, is a DIY, and lowers the freezing point somewhat, which you indicate is all you need. Dissolve it in the water first like your powder. It will freeze if it gets really cold, but since you don't use your tractor when it's below freezing out anyways, that shouldn't be a problem. I can't comment on any corrosion resistance, but that shouldn't be a serious problem since your rims were painted before the tires were mounted. Even just plain water would take many many years to rust through a steel wheel rim. The rotted rims you've seen are from the CaCl2 people historically used in tires.