I’ve done some digging and I found this resource. Though the approach taken here is already familiar to you, it is your best option. ViewportFrames would best suit this problem.
I’m assuming that what you are trying to do is to build your own shader in order to draw the reflections better than ViewportFrames can. However, this is not going to work. ViewportFrames are built-in and are heavily optimised. Their close integration with the game engine allows better and more extensive optimisation (GPU) than you will ever be able to do with custom Luau shaders.