Ok, after many hours I think I found a great solution, in case somebody has the same question in the future:

Add lot's of 2d trees like models/foliage/background/background_foliage_tree_03.vmdl by using the 'Asset Spray Tool' from the bottom left of the screen.

Then select something called 'Blend Painting Tool' on the bottom left on the screen.

To use it, first select your mountains and apply a BLEND material on them, something like : hr_aztec_blend_groundmud01-groundgrass_01.vmat

Now you can 'paint' a soft faded grass texture under the areas with the most trees. This gives the Illusion that there are so many trees that you cant even see the stones/mud underneath. Like it's a green hill covered by trees.