Who's fought Silverado/ Sierra brake problems? and won?

Poor boost does not sound like it... the pedal will typically feel harder and not go down as far with less boost.

Being this is bad on the first stop of the day, it sounds like air getting in the system, or the calipers moving back and gradually pushing too much fluid back into the MC. I'd mark where the fluid was in the MC after sitting overnight, jump in and stop a few times running, and re-check the MC fluid level to see if it dropped a lot as a first check.

Looks like the parking brake mechanism shouldn't move the rear caliper at all. So probably not there.