I'm no expert, but it's difficult to pinpoint exactly where the issue is, considering your particular hardware, the multiple distros, and the multiple guides used.

As I understand it, for a basic setup, you need to ensure that both the iGPU and dGPU are enabled in BIOS (and test them to make sure both are working). The main monitor connects to the iGPU (motherboard video out). The secondary monitor connects to the dGPU (graphics card video out). IOMMU and all virtualization options must be enabled in BIOS. Then you have to confirm the IOMMU groups in the OS, and if the dGPU is in its own group, then choose one of a few options available online to bind the dGPU to the vfio-pci driver. You may need to force / blacklist some modules along the way (will have to research those). Then you need to make sure that the VM is using UEFI, not BIOS.

Arch Wiki is a great tool for explaining this. You may also look for a guide that actually uses a Ryzen APU and an Nvidia GPU. For example, this one: https://ounapuu.ee/posts/2021/08/29/testing-gpu-passthrough-on-amd-ryzen-5700g/