I brought a cheap PCIe Asmedia to pass through to my TrueNAS, its important it has its own IOMMU group, I noticed from my board topology all CPU lane fed PCIe slots are fully isolated on IOMMU so use for that purpose. The onboard stuff shares so I keep them for the host.
You should be able to disable IOMMU grouping in the bios somewhere, which would then invalidate the IOMMU configuration, meaning if thats whats stopping you booted it would allow the boot to complete.