Technically there're just 3 layers: DXVK over MoltenVK over Metal. D3D8 and Vulkan here are just the APIs implemented by those adapter layers.

There're some D3D implementations over Metal that could skip the Vulkan layer but none implement the old D3D8 so you'd still need another layer that implements e.g. D3D8 over D3D9-11. Also, DXVK and MoltenVK have got a lot of traction and fixes on their own, so they're probably the most accurate pipeline for D3D on Metal.