I have 9,500 miles on my MXM4s, and I just measured the tread depth which varies from just under to just over 5/32". I rotated my tires once at 5,000 miles and the treadwear is very even on all four. Since the tires start at 8/32" of tread, it seems likely that my tires are wearing at about the same rate as yours. Because I need to drive it in unexpected thunderstorms at any time, I will replace the tires when they get down around 3/32" of tread, so I'll probably only get another 5 to 6 thousand miles out of them, and have to replace them at about 15,000 miles.I stretched my original MXM4 on the AWD to 19,000 by careful rotation and taking the front down to 2/32" and the rears right down to a shadow of tread left.
What I'd really like to do is get a set of the wheels that Tesla puts on the Chinese Model 3 Performance cars. They are 19" with aero caps, weigh only about a pound more than the 18" aeros, and are at least as efficient as the 18s. Then, since I run winter tires during the winter anyway, I'd get a more summer oriented tire for the 19s, maybe the PS4S.