How to retrieve the JSON string inside a controller and convert it in an array?

Question

I have a controller that returns a JSON on the page. This is the code

//Above in the code
injection of \Magento\Framework\Controller\Result\JsonFactory $resultJsonFactory


$result=[
    "orderId"      => $orderId,
    "status"       => $order->getStatus(),
    "grandTotal"   => number_format($order->getGrandTotal(),2),
    "totalFacture" => ...,
    "items"        => $arrItems,
];    

return $this->resultJsonFactory->create()->setData($result);

Then I . have a Plugin that intercepts the URL and I should retrieve this JSON and process it.

For now this is what I have:

public function afterExecute(\MyComp\MyMod\Controller\Test\GetOrderInfo $GetOrderInfo, $result)
   {

       //return $result; //Displays the JSON in the browser properly

       //Pass trough the JSON obj OR Retrieve the real JSON string
       //Not working !!  
       foreach ($result as $obj){
           echo $obj->orderId;
       }

       //Another attempt shows that $result is not a JSON object it self 
       $class_methods = get_class_methods($result);
       foreach ($class_methods as $method_name)
       {
          echo "$method_name<br/>";
       }
       //__construct
       //setData
       //setJsonData
       //setHttpResponseCode
       //setHeader
       //setStatusHeader
       //renderResult
       //___init
       //___callParent
       //__sleep
       //__wakeup

   }

I can't use json_decode because $result is not a string, enev if I see in the browser the expected JSON.

So, how could I retrieve that JSON string and convert it into array?