opacityMap would affect the rendering the mesh hence why there isn’t a ‘detail’ version of it.
@TonyLGC did this for their knife configurator: Showing multiple textures on the same model
@TonyLGC are you able to help here?
IIRC, you had to have a big enough border around the decal detail texture and set the texture to clamp so that the mipmapping of the texture doesn’t grab the pixels on the edge of the mipmaps and stretch them.
You can then use the UV offset and tiling properties to position the decal on the detail map.