I was able to do this in Blender, my goal is to have a smooth transition between two texture. I did this because I'd like to avoid having a huge 8k texture on the ground.

However, it is not possible to export this in a GLTF file, so I tried to do this in Three.js unsuccesfully. I have no clue what approach to take.

This is the best result I was able to get so far. This is by directly importing the .gltf from the screenshot above inside Three.js