Easiest way: Don’t use landscape.
And you can have near infinite biohmes.
When landscape pieces are just sculpted meshes you can paint whatever you want on it.
If you insist on using landscapes, then you have to design a master material with 3 paint layers.
In which you can change textures via instancing.
You then create different material instances for different biomes and you assing different materials to different landscape tiles.
When you paint, or create a splat map, you have the same 3 layer names. For instance
Dirt, grass, rock.
But on different level tiles they assume different looks, so dirt could look like grass or sand or dirt…
Realistically. Its a waste of anyone’s time. Just use meshes.