How do I recreate this Ghibli bush?

It has no texture or material, but it has a dark shading on some spots, I want to recreate this with a version of mine without triangles because the original mesh has like 18K Triangles and I do not want to use it (I need an explanation to do it, because just saying “bake” confuses me a bit as i am not too experienced with blender.)

In case anyone says “Just Decimate it”, well… I tried… Nothing looks good, even 7k Tris
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