Why?
Unity Supports OpenXR https://docs.unity3d.com/Packages/com.unity.xr.openxr@0.1/manual/index.html
StemVR now fully supports OpenXR so a separate build shouldn't be necessary https://store.steampowered.com/news/app/250820/view/3044967019267211914
This will solve all of the "Add Windows MR"...