Display a world-space canvas to overlay all the meshes but not the other UI objects

I have a MRTK (Mixed Reality) scene, with a 3D object and 2 world-space canvases; one to display the details of the 3D object, more like a speech-balloon and the other one is a Menu. The speech balloon canvas only has TMPro text gameobjects. Now I have to display the speech balloon in such a way that it should be overlaid over all the other meshes in the scene, but not over the other UI elements of the other canvas (The other canvas will always be in front of the speech balloon anyway). I tried using the Distance Field Overlay shader for the TMPro text; but it overlays not only all the other meshes but also the other (menu) canvas. I'm not an expert in shaders and all, so any suggestions are welcome.