Good news! After clearing seven passes of memtest+ with flying colors, I had cause to fuss in the bios for a while. It looks like something is promoting a bios reset, which has been reenabling PBO, Asus’ awful auto overclocking features, and messing with PCIe bifurcation. After getting the settings back to where I left them, the system has seemed much more stable. I’ve gotten two drives upsized and resilvered without any hitches, and temps remain nice and low
I do believe @joeschmuck called it right - temps were getting out of hand as the board tried to push the chip to boost too high. That’s what I (rightfully) get for not using a server oriented board