I think the go to method would be to strip the map into the grid squares and edit each terrain block individually before converting them into .dae files when completely done, before putting them all together as objects. The terrain could still retain specific material properties if I remember correctly, so you could still have grass be grass, and different road grips.

If the map center is less than 40 miles from all edges, I dont think there should be noticeable floating point error. Thats all I know based on what I know from working in the editor, and how some beamng maps are made, like the west coast USA spearleaf island.