Is it possible to upgrade the hard drive in the 2018 Mac Mini?

Nope, it's not possible. The flash storage chips are soldered to the logic board. You're stuck with what you get.

"The Samsung X5 SSD’s connected via TB3 are very fast. They’re NVMe drives…When I got the Mac Mini with an internal 256GB SSD, I knew that I’d be adding an external drive. Very happy to have found these Samsung drives, runs at the speed of an internal drive.

Where Apple really got me, though, was that I selected 8GB of memory, intending to upgrade the memory myself if I needed it. I had read that memory was user upgradeable. Well, when I saw what would be needed to do the upgrade myself, taking just about everything out of the case, I decided to have Apple do it. The labor costs were fine, but they really rip you off on the cost of the memory chips. I should have asked if they would do the upgrade using memory chips that I supplied, but decided not to go that route. I doubt that they would gone along with that, and missed out on the very high margin on the cost of the memory chips. Anyways, it cost about twice what it would have cost to get the memory at the time I configured the system on the Apple web site. A lesson learned :-)."

So How do I remove the soldered SSD and replace it with another? It does seem worth the effort.

"Can anyone who understand the Mac Mini logic board explain what to unsolder and to re-solder a PCIe NVME card to upgrade with a NVME SSD?

I know this one is possible.

Choose any best PCIe NVME Card. The best reliable and fastest transfer rate.

This is the only solution for the Drive upgrade in that Mac Mini 2018. Or, any other unsoldering & soldering method that will work perfectly.

Thanks"