So let’s do some math. You want 8km^2 size with 0.5m details. Minimal amount of storage space this need is (16*1024)^2 * 2bytes for heights = 512Mb. For rendering you just need few thousand indices to make grid and share vertices. Memory for this is negligible. At editor you can double this so you can edit and render it smoothly. You also need to store paint weights for this. First layer could be free. Second layer cost byte per sample. So 256Mb per layer after the first one. For editor you can double this. XY offset map is 512Mb but this should be optional. For absolute performance layers could use smaller texture than heights. To make bigger landscape than this you probably need streaming. But streaming should be simple. If only couple first LOD would load high detail heightmap and rest of the landscape could use single global height texture 1/16 of size.
Making this kind of system is quite simple but making good and robust tools are nightmare.