Why would you think that Kubota would put a loader on that tractor that the front axle cannot carry with the bucket fully loaded? 4WD isn't gonna hurt anything on unpaved surface other than chewing up the grass a little if you turn too sharp. I always use 4WD when pushing or pulling. Why risk spinning the tires? Spinning tires is for drifting, not tractoring. The front wheels, tires, and axle ratio are all designed so that the front end is trying to outrun the back end just a little. That's how it actually helps the rear tires. Not by enough so that it's constantly digging, but if you run 4WD on pavement a lot, you'll start to see accelerated wear on the front tires. Rule of thumb, if you're working on dirt, 4WD. IF you're cutting grass, or on pavement, 2WD. Gravel is just very grainy dirt.
Looking at that photo, I'd have had the drive done at the same time as the new slab for the garage. As flat as that is, it would have been simple to set the forms and do the grading and prep work yourself. That would have knocked out a big portion of the cost. It dropped the price of my slab for my shop from as high as $15K down to $3300. Plus, the lumber that I did the forms with was mine to repurpose in the shop construction. I'm a stingy tightwad, and it's more about saving the money, but if I don't have to spend more on structural lumber, that's eco-friendly too.
You're completely right to worry about gravel/sand on the tires on a sealed floor. That makes me wonder about the futility of putting an epoxy coating on that floor with a gravel drive approaching it. Concrete makes a lot more sense, especially if you want a squeaky clean garage floor. You cannot stop sand/dirt/rocks from coming in with your cars with anything other than a paved or concrete drive. Even then, you're going to get some sand/mud. Traction is what your tires are for, and better traction means they can hold bigger rocks. The tires on my truck (BF Goodrich All-Terrain) will pick up #47 gravel. Don't get too close behind me.