What's the problem with this Karabiner complex modification?
option should be defined as a modifier key. simultaneous is used for non-modifier keys. Besides, you'd better separate your mappings into a file from .config/karabiner/karabiner.json, 

~/.config/karabiner/assets/complex_modifications/delete-forward.json

{
  "title": "Delete Forward",
  "rules": [
    {
      "description": "Simultaneously press Option + Del to get a ForwardDel",
      "manipulators": [
        {
          "type": "basic",
          "from": {
            "key_code": "delete_or_backspace",
            "modifiers": {
              "mandatory":[
                "option"
              ]
            }
          },
          "to": [
            {
              "key_code": "delete_forward"
            }
          ]
        }
      ]
    }
  ]
}


Go to Karabiner-Elements Preferences -> Complex Modifications -> Rules, Click "Add rule". Then you'll see the new created rule from this separate file.