tire sizes for 18-wheelers - dumb questions

I want to lose the stock 43" 11.00x20 military tires and 2-piece Budd rims from my M818 5-ton military tractor and put one-piece Budd rims and tubeless tires on it. (the avatar shows 53" 16.00x20's but I got rid of them, didn't consider that safe) I hardly ever leave the pavement, a dirt/gravel road is about as far as I would go.

So yeah the main reason is highway safety and I know the service life will pay off. Weight is not much of an issue as the whole truck is only 24K Lbs and nothing will be hauled or towed, it's a hobby.

There are so many tire sizes and types for 18 wheelers that it's pretty confusing. Obvious steer tires for the front, but what's the deal between drive tires and trailer tires, they sort of look the same to me.

There are ten tires on the truck so if I match the circumference or height, I could do 2 or 4 at a time and sort of spread out the cost over time between the three axles.

1.) Are 20" or 22.5" rims and tires more costly or common? Where to find good used steel rims? (no chrome or aluminum)

2.) Where can I find charts showing tire diameter or circumference?

3.) Any advice on big-truck tires in general? I'm sure this is the place to ask what to avoid, what works well, etc.

Thanks!