That pad thickness looks fine. Of course, I'm assuming the other three pads on the rear brakes look the same.
My 2013 rear brakes groan, too. Currently, my 5 has 78,200 miles.
At 58,600 miles I replaced my rear pads. One pad wore out much too quickly before the other three did. It irked me, but I think I was doing too much parking brake slow downs and the 5's rear disc brakes don't respond well to that.
Back in early August 2017 (perhaps 73,000 miles) I changed out my rear disc rotors because one was warped and had a pulsing groaning sound when the brakes were lightly pressed while the car was moving. Super annoying.
Just this past week, I noticed the groaning is starting to return again. But it's an even groan and very slight so far.
The best I can tell is that the rotors have to be perfect and the pads have to be new for the 5 to not make groaning noises.