I finally fixed my i218v in the NUC that I have -- I contacted Intel support and they agreed with my assessment which was that it was bad hardware. I had done extensive testing, replacing everything in my network, and trying with a USB NIC, updating drivers, etc. All signs pointed to it being hardware.
They replaced the NUC under warranty, and sure enough, it has worked flawlessly since. It was a huge headache -- years of diagnosing and troubleshooting -- but in the end, Intel came through. If you're covered by warranty and have done some similar troubleshooting, maybe you have a bad one too?