Thanks for the review. I recently put together a designare X399/1950x build thanks to the recommendations on this site.
Gigabyte hasn’t updated the bios since October, so it is unlikely that there have been changes to IOMMU.
The Noctua 14” is great and does not interfere with the first PCIE slot (unlike zenith extreme). The noctua has 3 and 6mm offsets to give more clearance for the pcie slot - either allows enough room. It is a tight fit for RAM, but if you move the cpu fan up there will be enough room to populate all slots.
I have 64gb of RAM running at 2400 with multibit ECC in both Linux+win10 (4x CT16G4WFD824A.18FB1 from QVL ). There is no 128gb ECC on the QVL list. Getting the system to recognize even 4 sticks was a PITA. Ultimately I had to install 1 stick in slot indicated in manual, reboot, install 2nd stick, reboot, etc… to get all recognized.
PS- I do not have the fractal case so I cannot speak to heat sink clearance
PPS- one additional note about RAM: at first the system would intermittently “lose” one of the 16gb sticks. Reseating the CPU in the socket seems to have fixed it. I was surprised that worked but I can’t argue with results.