I’m sorry but I have no time to test your code ATM. I know it’s not exactly what you are trying to do, but what if you pass the light position to the fragment shader and then simulate a 2D glow by increasing the RGB intensities if they are close to the light position? It would not be a volumetric light, but it can be good enough in some cases…
Otherwise, maybe https://discourse.threejs.org/tags/shaders has more experts in this area?