There's just a couple of minor issues in your playbook

1. You are loading the json data from the file as a string, rather than json/dict. Instead of your command/cat task, use this to load the data in:

    - set_fact:
        surveySpec: "{{ lookup('file', sharedDataPathFile) | from_json }}"
   

2. The other problem you would have hit is you are attempting to update just the choices value (i.e. a string), rather than the choices dict item. Just a small change required in your json_modify pointer:

    - json_modify:
        data: "{{ surveySpec }}"
        pointer: "/spec/5"
        action: update
        update: "{{ new_choices }}"
      register: result
   

One other thing .. the array index is 5, not 6

That is a simple case and you can use what @JGK has replied with, or you could just define a list and add those json elements to it. Without any modules!

i.e.:

  - vars:
      elem:
      - {"firstname": "John", "lastname": "Smith", "user": "john"}
      - {"firstname": "John", "lastname": "Smith", "user": "john"}
      - {"firstname": "John", "lastname": "Smith", "user": "john"}

  - set_fact:
      my_list: []

  - name: add to list
    set_fact:
      my_list: "{{ my_list + [item] }}"
    loop: "{{ elem }}"

  - debug:
      msg: "{{ my_list }}"

  - copy:
      content: "{{ my_list }}"
      dest: ./x.json
    delegate_to: localhost

and here's the result validated with json python module or jq:

$ jq . x.json
[
  {
    "lastname": "Smith",
    "user": "john",
    "firstname": "John"
  },
  {
    "lastname": "Smith",
    "user": "john",
    "firstname": "John"
  },
  {
    "lastname": "Smith",
    "user": "john",
    "firstname": "John"
  }
]

Python:

$ python -m json.tool x.json 
[
    {
        "firstname": "John",
        "lastname": "Smith",
        "user": "john"
    },
    {
        "firstname": "John",
        "lastname": "Smith",
        "user": "john"
    },
    {
        "firstname": "John",
        "lastname": "Smith",
        "user": "john"
    }
]