For overlaying UI stuff that should appear in front of everything else (but has to be within the world scene), I usually use a separate camera with a higher Depth than the main camera and set that separate camera with Depth Only for its Clear Flags. I set that camera with a culling mask that only display layers with the stuff I need to display in it and turn those layers off on the main camera. (You can set that secondary camera as the children of the main camera if you move the main around.) You can set that secondary UI camera as the camera for your World Space rendering UI.