I know this is old, but the easiest way I can think of is to use multiple tilemaps with different tile sizes. For example, create two tilemap sibling nodes. If your tiles are 32x32, one tilemap will have all your tiles at that size, the other tilemap will have a tile size of (328)x(328). In this larger tileset you can make one tile that consists of your eight tiles in a sequence (keep the rest transparent). You can also do this for other patterns.
With these two tilemaps overlayed, it will give the illusion that all the tiles are a uniform size from the same set.