Great point on going smaller. The Tire Rack calculator for the Limited didn't offer anything smaller than 18", probably for a reason. The new Ram DT has issues with caliper clearance on many of the 18s, even though the Rebel comes with 18s, precisely due to caliper clearance. The wheels require a barrel that gets larger on the inside of the wheel for clearance.
You might find a Ford forum where folks are discussing the Explorer and see what they're experiences have been with wheel and tire combinations?
275/60-20 was the size that the Discount Tire calculator kept telling me was OEM for the 2019 Ram DT. It is actually 275/55-20 and there are fortunately a ton of options in that size.
As to unusual sizing, the 2016 Mercedes GL 450 that I had was so unusual at the time that there were only like 3 tire options in that size. Having a size that makes the tire almost a 'commodity item' certainly keeps the pricing down.