Ive got the RE050As also and ive got 10.5k so far, down to 3-4 mm across the back, excluding that stupid half depth outer ring, and 4-5 mm on the front again excluding that half depth outer ring.
Tbh the rules say its 3/4 of the tread, so i think its ok to exclude the outer ring. As for them wearing quicker on the outsides, yh they do but you also need to remember that the most outer groove starts off at a lower depth than the rest of the tread, and it also has no markers on it to give an indication of depth remaining for the outer most groove.
Over the summer it wont matter too much, its only the standing water that screws you if you dont have enough tread you will aquaplane, but on a nice dry day there are no issues with shallow tread, as long as they are legal ofc.