From what *I remember*, I believe I once watched a REWA video regarding the NAND in iPhones and what typical indicators you can look for to account for failure of the NAND, and I think anything between 17%-20% flash progress stalling in 3U indicates general NAND failure. That would be my guess, but beyond that give it a shot to put the motherboard under the microscope and peel the heat transfer tape off the board and look for any obvious component damage. After that, if you have one, try looking for shorts on the board via a thermal camera.