This sounds like a really interesting thing that you are trying to do. I had a look at the JSON you provided and I am guessing that it may not be fully fleshed out and was only meant as a rough example. In particular, ManyChat may not like the empty arrays. I would suggest fleshing things out a bit more like this:

  "version": "v2",
  "content": {
    "type": "instagram",
    "messages": [
      {
        "type": "cards",
        "elements": [
          {
            "title": "Sample Product",
            "subtitle": "This is a sample card showcasing a product.",
            "image_url": "https://dummyimage.com/600x400/000/fff.png&text=Sample+Image",
            "action_url": "https://example.com/product",
            "buttons": [
              {
                "type": "web_url",
                "url": "https://example.com/product",
                "title": "View Product"
              },
              {
                "type": "web_url",
                "url": "https://example.com/cart",
                "title": "Add to Cart"
              }
            ]
          }
        ],
        "image_aspect_ratio": "horizontal"
      }
    ],
    "actions": [
      {
        "type": "web_url",
        "url": "https://example.com/shop",
        "title": "Visit Shop"
      }
    ],
    "quick_replies": [
      {
        "title": "More Products",
        "payload": "MORE_PRODUCTS"
      },
      {
        "title": "Contact Support",
        "payload": "CONTACT_SUPPORT"
      }
    ]
  }
}

See if that works for a start. If it does, you should be on the right track and can continue to get your real data into a shape that emulates this.

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Ted - Zapier Support