SSR works fine on SLW materials. You don’t set it inside the material, SLW adheres to whatever the global reflection method setting is, which you can set in project settings and/or the post process volume.
Like I said, as soon as the object is off screen it will return to black. Nothing you can do about that without the surface cache.
If you have enabled Nanite and they are still black then it may be that the surface cache is being culled because the meshes are too small. You can adjust the Lumen Scene Detail setting in the post process volume and it will be less aggressive with the surface cache culling, this will increase the cost though.