That’s not quite the right workflow for that object.
First–you’re using a high-resolution mesh, which should not be the mesh that you use in the game, you would make a much simpler mesh from that without stuff like the rounded edges and a much lower polygon count. You would then unwrap your UV’s for your low-poly mesh and then bake a normal map from the high poly mesh to the low-poly mesh. That would make it look very similar to the high poly mesh but it would have much better performance. After that, you would do the texturing. For something like that you don’t really need Quixel to do the texturing, Quixel and Substance most benefit you when you’re doing something like worn and weathered materials, so if this object was very old and had scratched paint and edges then that’s what you would use Quixel for. Since you’re just doing basic colors you can just bake your normal map and then do basic texturing in Photoshop.
Also, it would help if you worked on your UV’s to have fewer islands, especially if you’re going to have the object as a static mesh in UE4 since it will require baked lighting.