Lightmap for openworld

My terrain is literally a flat plane that is enormous. It takes about 30 minutes for the player to get from one side to the other. The entire map has to be visible from anywhere since the player is up 25,000 meters up in the air, as if they are looking out and observing.

I want to bake lighting but I’m not sure if I should create one light map for the whole terrain, or split it into more manageable pieces. I’m not too concerned with the quality of the bake or the texture, this is more about really saving memory and improving FPS. Also, do I need reflection capture actors? There isn’t actually any reflections at all, but if they are needed for the light map I can put them in there, but I would have to have hundreds or more to cover the whole terrain. The same goes for the lightmass importance volumes, do I need multiple or just one large one?