It is my understanding that Nanite cannot handle landscapes because it is one big mesh. What about breaking landscape up into individual sqaure static mesh that Nanite can handle. As you sculpt the landscape whatever sqaure you are on an additional sqaure is selected on each side and then that mesh is combined to one mesh that you can sculpt. As you sculpt around the squares are combined and then uncombined so ultimately only the part you are sculpting is dynamically combined and then as you move off it reverts to static squares.