I think you entirely miss the point.
IF the flat png used for the “Grass” layer is called for, but you are in the snow biome, are you going to display the Grass layer? No… your grass layer becomes the snow. You can therefore swap (or rather than swap use the png on a different layer) grass textures with snow textures.
Now, when you work outside the engine and produce the masks, you just need to keep in mind HOW you blend it.
Very commonly, one layer is exactly the same across all tiles and is used for blending, while the others are instanced separately (by changing the images in the material along with other parameters your master landscape material allows for).
Personally, I use QGis to produce the base layers on which to improve upon.
If you have world machine, I’m sure it can also produce the “famous” splat maps that people often wonder about in formums.
If you use blender you can paint an auto material and bake out the different layers at the appropriate resolution. Mind you that this isn’t the same as Qgis accuracy or World Machine, but as a starting point it’s rather effective.