UPDATE: finally picked up my car TODAY. Problem solved (thanks to google, not dealership).
After installing some sort of insulators, ripping apart the cowl, replacing struts... still no improvement. So I googled to death and found a description that ticked all the boxes to a T (see below). One new engine mount #4 later... voila, no more noise. Finally. At least I got some new free struts out of the ordeal. Hope this saves someone some hassle!
NHTSA ID Number: 10099442
Manufacturer Communication Number: SA-039-17
Summary
SOME CUSTOMERS MAY COMPLAIN OF A CREAKING NOISE WHEN DRIVING THROUGH A DIP OR BUMP IN THE ROAD WHILE ACCELERATING. THIS OCCURS WHEN THE VEHICLE IS COLD AND CANNOT BE DUPLICATED ONCE THE VEHICLE IS AT NORMAL OPERATING TEMPERATURE. THE CREAKING NOISE MAY BE CAUSED BY THE #4 ENGINE MOUNT. THE NOISE CAN EASILY BE MISTAKEN FOR SUSPENSION RELATED COMPONENTS AND IS VERY DIFFICULT TO DIAGNOSE.
https://mazda.oemdtc.com/1279/creak...g-through-a-dip-or-over-bumps-2016-mazda-cx-5