How to blend between object ( i.e.. mountains with map)

Hi everyone,

I am creating strategy game. Something similar to CIV6 and AOE4 in one game. I have huge map with is loaded from server, it is procedural, and it is based on hexagons. My problem is:

I want to add mountains and cliffs ... this objects are more vertical so i would be better to have dedicated texture for this objects (not built this objects in map mesh). I want to have connection seamless like in other games (CIV, Old World, Humankind) and this is possible but I would have to extract vertex normals from terrain map and blend them with mountains this will be so expensive.

Do You have any ideas how can I achieve such an effect in the most efficient way.

Or generally how to create map for this type of games.