If you have a late 2014 Mac Mini that is grinding to a halt due to the tiny 4 GB's of SOLDERED ram (what were apple thinking!) don't give up on it. I was about to get rid of it as it was becoming unworkable but after a bit of research found out that it does have a PCIe connection that is capable of working with NVMe, to support a tiny SSD.
The upgrade is now complete and it's all working great. I did have a few hiccups as i'm using non-standard apple parts. The original SSD was a Samsung PM981, this crashed the system but swapped to the cheaper MP961 and its working great.
I now hopefully have a few more years of iOS development in this box. If you would like to do the same, you will need the following...
Mac Mini A1347 2014 2015 SSD PCIe flex Cable Connector Adapter
SAMSUNG 256GB NVMe M.2 SSD PM961
M Key NGFF M.2 NVME SSD converter adapter card
and finally, don't forget a T6 security Screwdriver.