I prefer simplicity and using the first example but I'd be happy to hear other options. Here's a few examples:

HTTP/1.1 403 POST /endpoint
{ "message": "Unauthorized access" }

HTTP/1.1 403 POST /endpoint
Unauthorized access (no json)

HTTP/1.1 403 POST /endpoint
{ "error": "Unauthorized access" }

HTTP/1.1 403 POST /endpoint
{
  "code": "UNAUTHORIZED",
  "message": "Unauthorized access",
}

HTTP/1.1 200 (🤡) POST /endpoint
{
  "error": true,
  "message": "Unauthorized access",
}

HTTP/1.1 403 POST /endpoint
{
  "status": 403,
  "code": "UNAUTHORIZED",
  "message": "Unauthorized access",
}

Or your own example.