Customise emitted SNS message in state machine

I have the following state machine step:

    "Step Name": {
      "Type": "Task",
      "Resource": "arn:aws:states:::sns:publish",
      "Parameters": {
        "TopicArn": "${TopicARN}",
        "Message.$": "States.JsonToString($)",
        "MessageAttributes": {
          "type": {
            "DataType": "String",
            "StringValue": "$.type"
          },
          "id": {
            "DataType": "String",
            "StringValue": "$.id"
          }
        }
      },
      "End": true
    }

which works fine but I'd like to customise the body of the SNS message, somehow like this:

    "Step Name": {
      "Type": "Task",
      "Resource": "arn:aws:states:::sns:publish",
      "Parameters": {
        "TopicArn": "${TopicARN}",
        "Message.$": {
          "id": "$.id",
          "param1": "$.param1",
          "param2": "$.param2",
          "metadata": "$"
        },
        "MessageAttributes": {
          "type": {
            "DataType": "String",
            "StringValue": "$.type"
          },
          "id": {
            "DataType": "String",
            "StringValue": "$.id"
          }
        }
      },
      "End": true
    }

However it does not work because Message has to be a String. I was hoping to wrap my object in States.JsonToString but that also doesn't seem to work.

Is there a way to achieve this or will I have to resort to using a lambda instead of the SNS integration?