My instinct is this:
You would probably get a lot better performance if, at startup when you calculate all those points, you just create a procedural mesh which basically mimics the terrain, i.e. a big tessellated quad with the vertices all set to the heights you calculate. Then, just attach that mesh to the camera and assign a material that has a grid pattern with depth test off, or use a scene capture actor to render it to a texture if you really want to use it inside of UMG. Depending on the size of the terrain and/or the density of the height samples you take, you may want to have the procedural mesh represent a local area around the player and modify the vertex heights as you go.
This is something I wish Unreal had better support for, by the way. From a workflow point of view, what you’d LIKE to be able to do is just create a scene capture actor and set it to render only the terrain with a custom material applied to it (one that shows a grid texture). Then you could attach that scene capture actor to your character with some Z offset looking down at the terrain and you’d get exactly what you want. The support for rendering subsets of actors into a texture with material swaps applied is really lacking IMO. Even if lighting weren’t supported when using scene captures in this way, it would be infinitely useful. I would also add that this is something more easily done in competing engines.