Thanks for everyone’s suggestions!!
So I figured it out in case it’s helpful to anyone - super simple, the VRMs weren’t getting chilled enough… I hadn’t completed my build and while I had watercooled both CPU and GPU, and was doing my leak testing etc., but had no fans setup at the front of the case (I figured the VRMs were doing fine since it was a pretty chilly room, open air with the sides off the case, but it definitely was not enough). The heatsink situation is pretty weak on the ROMED8U-2T, but once I placed fans blowing directly on the VRMs - Monero mining is now pegged at 48K, passmarks are more consistent around 80K etc.
The other big heads-up/regret with going mATX for anyone out there trying to do a small/sexy server build - the right slimsas to u.2 cables are incredibly hard to find (probably everyone knows this on this forum), and I had to order from serialcables with a 3-4 week lead time… and hopefully they work since the connector needs to be much wider than the other slimsas cables I have. Related, the SAS to Sata breakout of course only works for 6g sata drives, which I knew before I purchased, but it is turning to be more of a frustrating issue since the slimsas cables are so hard to get. Plus, now I wish I went dual CPU since the one is performing rock solid now
4.8min chia plots in ubuntu, while leaving a few cores for the OS!