That’s about it for your options. Outside of cracking open a thick book of graphics programming and making an UE4 Engine Plugin, there’s no way to only take a one frame render of a scene/object. There are only two components available for this kind of thing provided by the engine: the Scene Capture 2D component and Scene Capture Cube. Both of these options will render every frame and are very expensive (Scene Capture Cube being 6 times more expensive than Scene Capture 2D).
My suggestion, if you have the experience, would to be make the plugin and make it available to the community (everyone would eat this up!). Otherwise if a rendered icon is necessary, You could use the Scene Capture 2D component. But when ever you display this icon on your screen, I would design your UI so that you only have a very limited number of these rendering objects displayed at one time, destroying them/disabling them whenever they are not in view through an interest management system. Do this while keeping in mind your target platform and its specs as well as using the profiler to see how expensive this is going to be.