Make a material with texture parameters and 3 or 4 layes.
on 4 adjacent tiles you change the instance and customize at least one texture for each.
Assume A, B, C, D. Using 3 texture for example.
Default mat is grass, mud, rock.
A - sand, grass, rock. blends into B-C
B - forest, grass, rock. Blends into A-D
C - grass, mud, snow. Blends into A-D
D - grass, mud, rock. Blends into B-C
assume they are tiled as
A needs a layer that can blend into both B and C.
B needs a layer that can blend into A and D. Etc.
If you bump up to 5 layers with some careful paint work you can make the whole landscape have a multitude of different textures for a very little cost. Definitely much less than having 15/20 layers within 1 material.
To apply the instances you open up the level - select unlit if you need to see. Apply the new instance.
You then return to the worldcomp level, load only that one level and paint. Load the adjacent level after save/unload and paint.
You need to make sure that the same layer has the same texture since the painting can otherwise be problematic when you load multiple levels. Regardless of that, it works. And obviously it works best when you import paint layers done in a 3rd party application.