You may need manually hex edit of AMIsetup modules, read a bit: msi z490 unify bios mod - BIOS/UEFI Modding / BIOS Modding Guides and Problems - Win-Raid Forum (level1techs.com)
3 topics to read
https://winraid.level1techs.com/t/guide-adding-bifurcation-support-to-asus-x79-uefi-bios
https://winraid.level1techs.com/t/guide-how-to-bifurcate-a-pci-e-slot
https://winraid.level1techs.com/t/guide-pcie-bifurcation-mapping-iio-iou-to-pcie-slot