If it becomes “flat,” then maybe the mesh just isn’t tesselated enough?
Displacement only displaces vertices that actually exist. Unreal Engine doesn’t generate tesselated additional triangles on regular meshes. (I seem to recall that older versions used to, but it was problematic and got dropped.)
I think the best option for you, is to massively tesselate your model, and add all the variation you need in the model, and then turn on Nanite so the model runs at good performance. The point of Nanite is not “make flat models look good,” but instead, “make very detailed models run fast.”