It doesn’t really make sense to use pixel depth offset with decals when you think about it. Pixel depth offset is designed to reveal accurate intersection with other geometry. So if you have a solid floor or wall and put a decal (or floating plane) and then use pixel depth offset to push the pixels down, by definition you will reveal the underlying floor or wall, unless you float the decal or plane away from the surface as you found.
If you want a decal that can appear to make a hole I think that will be some completely different like depth only writes or something. I am not sure what it would be honestly.
What do you actually need the pixel depth to change for if its a decal? It will only be noticeable if other objects need to actually appear to go inside the hole for some reason. It should look just as good without pixel depth offset.