Where do I store my terrain (open world)?

I’m using level streaming to load/unload sections of a pretty big world. I’ve been advised to store all of the terrain maps in the persistent level, so the entire world is always open but otherwise empty until the other objects are loaded in via streamed levels. However as the terrain is so big this doesn’t seem to make logical sense. Can I instead have additional level maps just for big (but not the whole) sections of map, counties maybe, that are loaded at a ‘border’ via an BeginPlay event and remain open alongside the smaller area levels and persistent level until the player reaches another county border? Would that make more sense?