Cloned my tester install over to a 250GB Samsung 860 Evo. Unsurprisingly, massive responsiveness improvement. My view of Mojave so far is that whatever front-end stuff it’s done is relatively minor. The back-end stuff is unobtrusive. My prediction in the original post of this thread is correct: this just feels like one more minor update. Which is fine by me, because it means I was correct in presuming that dosdude1 would succeed. He did. This is almost an 11 year old computer that is still kicking. Now that the problem RAM was removed, there appears to be no issues with the Mac Pro 3,1 continuing to run. I’m very pleased, indeed.
Update: My GTX 660 works without webdrivers on Mojave. It didn’t work at all on High Sierra. But it had worked on Sierra. Glad to see this got fixed.