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...
and finally, don't forget a T6 security Screwdriver.