Hey, Have you thought about a faulty logic board? either that or a faulty drive like you briefly mentioned in the OP. Just tryna help. Here is a tip, If you have access to a Windows 7-8-10 PC, download Microsoft’s ISO Burn tool, put the ISO on a jump drive, and boot your Mac Mini into windows using Bootcamp. At this point you could run a favorite program of mine called ‘Task Manager’ and check your RAM, GPU, CPU, Drives, and network info, and cross-check them with the spec the Mac had when new. this will uncover dead NAND cells in the SSD through the minor loss of storage and possibly dead RAM modules (RAM Cells/Chips, not the whole stick.) and help out with troubleshooting. if everything is fine, i don’t know what else, but i think there are bad sectors in all the ram (PRAM, DRAM, And bad GPU RAM in the extreme case.) and SSD. hit me back if you figure it out. Good luck.