Ok, I think modifying the voltages helps. I'll post my settings later.
However, it now appears that the RAM is getting too hot (under full load, even with heat spreaders), causing WHEA errors. I'm ordering a new case, and I'm going to install RAM fans to help cool it. The Ryujin II 360 appears insufficient in cooling capacity.
I'm also ordering a new PSU, Corsair 1600 watt, as I've read that this board is very touchy when it comes to voltages.
And I'm replacing the Mushkin 8TB NVME drives with Sabrent versions.