Curious why you would entertain the thought of buying a taller tire for your spare (285/75r17) when your 4 main tires aren't going to get any taller with wear. Finding a used 255/80r17 with half-tread would come closer to matching the size of your new tires across their full lifespan. Also, the width of the 285 will make it hang lower and be more vulnerable to damage. Unless you have a flat while your tires are new, or you buy a matching wheel and tire so you can do 5 tire rotations, your spare will never be the same diameter as your other tires. Also consider, the 285/75r17 will be heavier than than the 255/80r17 - Toyo AT III is a 7 lb difference. It isn't much, but it's that much more to wrestle out from under the truck. Plus, it's extra weight you're hauling around all the time, impacting fuel economy and reducing remaining payload. The thing you have to consider most with respect to diameter is the G80 locker; too much difference in diameter and the spare will need to go on the front. I never worried much about the 1.1" difference between my OE spare and the 275/70r17 tires I was running on my 2014 Sierra; I just knew that if I had a flat before my tires were pretty near worn out, the spare would have to go on the front.
How often have you had to use the spare tire on your vehicle? For me, it's been 3 times in the past 22 years. Off road, I'd prefer to repair and re-inflate if possible - likely quicker and safer than jacking up a vehicle on uneven terrain. Just my thoughts.