3900X, 4 dimms, 3 physical 16x pcie, good IOMMU motherboard recomendation

Looks like AGESA 1.0.0.3 does address most of the x370/x470 motherboard issues, if your choice of vendor is providing one yet.

Will say, the x470 Taichi Ultimate with the P3.30 BIOS works well now, except for the memory overclocking. Guessing infinity fabric shenanigans