Maybe the Movie Render Queue High Resolution function is what you are looking for. It splits the render into several tiles, renders the tiles one after another, then it combines all tiles to get the final image. It was introduced to create renders in way bigger resolutions than your gpu could normally handle, since with that the gpu has only to deal with a small part of the whole image.
Of course, it comes with some limitations (like several post processes do not work with it, or autoexposure) , not sure, if they have improved this since it´s introduction, or were able to lift some of those limitations for 4.27 or 5.0.
The problem i see with your method is probably the same, that the MRQ has. With only a tile (or a layer in your case) of the whole image, some effects are probably hard to apply correctly, since they would require information from regions of the image, that are in different tile or layer.