How to create nested json file based on user inputs?

We are working on building a product. Please see below. Here each entity on left panel is a collection in mongo db. Once the user completes filling all the forms details, need to create a json file with key-value pairs. This can be nested to any level. One rootMenuItem can have multiple MenuItems. And one menuItem can have multiple children. And so on.

This is UI page developed in angular Js. Each collection say rootmenuItem, menuItem etc will have save button to save the key value pair

{
  "version": "19.22.02",
  "constants": {},
  "gui": {
    "screenSaver": {
      "imageId": "ACU_screensaver/MBAC_black"
    },
    "menu": [
      {
        "configId": "test",
        "tets": "RootMenuItem",
        "icon": {
          "imageId": "teste"
        },
        "primaryLabel": {
          "rawText": "test",
          "textId": "tese.0216"
        },
        "intro": {
          "imageLayers": [
            {
              "imageId": "test"
            }
          ]
        },
        "children": [
          {
            "widgetType": "test",
            "configId": "21e88e",
            "children": [
              {
                "widgetType": "Tile",
                "primaryLabel": {
                  "textId": "test",
                  "rawText": "test"
                },
                "secondaryLabel": {
                  "bindingType": "test",
                  "deviceId": "test",
                  "serviceId": 1,
                  "serviceType": 40,
                  "ifTrue": {
                    "textId": "test",
                    "rawText": "An"
                  },
                  "ifFalse": {
                    "textId": "acu1.0236",

Multiple users can create and download this json file

We are using java spring boot micro services with mongodb

The problem here is how to create a json file from this data. Please check the sample json as well. What are the best strategies to generate json faster and store the file on mongo db.