Looks like you tried a lot of stuff, but I am missing a few things here:
1) it seems like you are running a memory only stress test, and judging by CPU frequency. Nobody knows what this test does exactly, so reduced CPU frequency might actually be normal
2) Check the sensor values. In particular, temperatures for CPU, CPU VRM, memory and memory VRM.
3) last not least: RDIMM can run hot. And modules with higher capacity tend to run even hotter. If this is in a workstation case without any special cooling for the RAM, 16x64GB is very likely to overheat during a memory stress-test. I have experienced this first-hand with a nearly identical system. 81°C is the threshold for DDR4, any more than that, and something will throttle. Again, looking at the sensor values would help.
You can either install this software from Supermicro:
SuperDoctor® 5 (SD5) | Supermicro
Or connect another system to the IPMI port.