is there a working example of those country layers somewhere?

Thanks!

When you need to overlay a single image on a map, the best way to do it is on the shader side.

This way has two huge advantages:
1. It's really fast.
2. It's elevation friendly.

Disadvantage:
You can only have one such overlay at a time.

Requirement: Tileset Control.

How to use it:
1. Download the script and shader for your Render Pipeline.
Built-in RP - TilesetWithOverlay.
URP/HDRP/Custom RP - TilesetWithOverlay Graph.
2. Import this into the project.
3. Add TilesetOverlayUsingShader component to the map GameObject, and configure the fields.
4. Set the shader to Tileset / Materials & Shaders / Tileset Shader.