Referencing variables in Step Functions

In my particular case I am trying to reference a value from the originating event that kicks off my step function the body of an SES email.

For example, this is the originating input event:

{
   "eventType":"connected",
   "GatewayEui":"abcd12345",
   "timestamp":"2022-06-14T19:40:17.511Z"
}

And the SES Send Email node is configured like so:

{
  "ConfigurationSetName": "test",
  "Source": "noreply@abc.abc",
  "Destination": {
    "ToAddresses": [
      "johndoe@abc.def"
    ]
  },
  "Message": {
    "Subject": {
      "Data": "Device disconnected"
    },
    "Body": {
      "Html": {
        "Data": "$.GatewayEui is disconnected",
        "Charset": "UTF-8"
      
....

This doesn't work - but notice I am trying to get the GatewayEui and reference it in the body of the email, but confusingly the only way I can figure out how to reach it is as as a JSON key/value. But all I want is the value.

How can I achieve this?

Also, two HUGE feature requests:

1. Please let us add in an example event JSON into the visual workflow so we can inspect how it would be referenced in particular nodes while we're editing the flow.
2. Please add a debug node! Which would let us log specific branches of a flow and not others.