How do you structure the "large distance" grid? Is it simply regarding not 100% passable areas as non passable? Or do you calculate a passability value for each "subgrid". (you could make a short-distance pathfidning-testrun from each of the 4 sides "side-middle"points to each other 3 middle points to check if there is a connection) Or use a floodfill to see if each side is beeing touched...