The only problem with drilling the pocket corners first is that you need a really fast spindle to make a nice hole in aluminum. ... up to 18,000 rpm would be nice.
If I had only ~6,000 rpm available, I might hog out the pocket with big indexable 2flute drills of a diameter nearly the smaller pocket side dimension, then broach the remainder of the pocket and the corners.
OR just let the CNC owner decide how to make the moderately deep through pockets with moderately difficult corner radii. There are a near infinite number of ways to do it with canned cycles, and shortest cut time may not be optimal for cost, considering cutter and machine availability, chip removal, presence or absence of an attendant, etc.
I.e., just send it out for quotes, with a full set of dimensions and tolerances please, and see what you get back.
Mike Halloran
Pembroke Pines, FL, USA