My opinion on filling to 50%: Make sure to put tubes into the tires. The additional presence of oxygen in the air above the liquid
ballast would seem to provide a perfect environment for corrosion. (Humid air in a continuously wetted environment) Having a tube would prevent that.
50% fill would give a smoother ride than 75%, because of the larger air cushion. The tractor won't be more stable with 50% fill as opposed to 75% in a static situation.
If it were me I would put the rear blade on, and see how that worked. Filled tires will be a nuisance to change by yourself. I'm all for cheap, and am possibly the cheapest person in the Yanmar forum in terms of what I think is a reasonable price for something, but filled tires doesn't seem like a good match for what you want. Wheel weights would swap more easily and safely between your different sets of tires.
Since you're talking about using it in the snow, you're going to need something other than straight water, which I'm able to get away with. That means you shouldn't just drain it onto the ground, whether it's windshield washer fluid (methanol) or antifreeze. Rimguard (beet juice) would be fine to drain, if you didn't mind the juicy mess. Having said all that, I would suggest trying to change the tires by yourself and then decide on your own; you'll know a lot more about your situation than some random guy on the internet blabbing on and on, like me. :laughing:
You may decide that if you filled your tires and added wheel weights you don't need to change them anyway. The little bit I've used turf tires in mud/soft soil I was disappointed, but even a single impromptu tire chain helped significantly. If your property has areas where you need to do a lot of work in mud, you'll want something other than turf tires. You know your needs best, and may be forced to experiment and discover what works best for you.