Getting heightmap from Camera

Question

I have a scene made of multiple planes and all kind of environment objects. I would need to create a heightmap texture of that scene to be used later so it should be achieved in edit mode (Example below is in runtime mode for development purpose).

So far I am able to capture the camera view but it's the normal result:

public Camera m_camera;

private void Start()
{
    m_camera.depthTextureMode = DepthTextureMode.Depth;
}
private void Update()
{
    if (Input.GetKeyDown(KeyCode.A)) 
    {
        var rt = new RenderTexture(512, 256, 16,RenderTextureFormat.ARGB32);
        rt.Create();
        m_camera.targetTexture = rt;
        m_camera.Render();
        m_camera.targetTexture = null;

        RenderTexture.active = rt;
        Texture2D tex = new Texture2D(rt.width, rt.height, TextureFormat.RGB24, false);
        tex.ReadPixels(new Rect(0, 0 , rt.width, rt.height), 0, 0);
        RenderTexture.active = null;
        byte[] bytes = tex.EncodeToPNG();
        string path = Application.dataPath + "/heightmap.png";
        File.WriteAllBytes(path, bytes);
        AssetDatabase.Refresh();
    }
}

Setting depth texture mode would allow to access the depth texture from shader but I am thinking there should be a way to use post processing script.

Not all objects in the scene have collider so I cannot use raycast to create it.