How to send an existing email to a new thread using gmail API

I'm attempting to send an existing email to another thread using the gmail API in Flask.

Context: I've created a new message object and successfully made the In-Reply-To, References and threadId equal the message_id and thread_id respectively. (Following Gmail's API reference)

def create_message(sender, to, subject, thread_id, message_id, message_text, service):
  """Create a message for an email.

  Args:
    sender: Email address of the sender.
    to: Email address of the receiver.
    subject: The subject of the email message.
    message_text: The text of the email message.

  Returns:
    An object containing a base64url encoded email object.
  """
  if type(message_text) != str:
    message_text = str(message_text)

  message = MIMEText(message_text)
  message['to'] = to
  message['from'] = sender
  message['subject'] = subject
  message['In-Reply-To'] = message_id
  message['References'] = message_id
  message['threadId'] = thread_id
  print('created message threadId: %s does == ..' %message['threadId'])
  print(thread_id)

  raw = base64.urlsafe_b64encode(message.as_bytes())
  raw = raw.decode()
  body = {'raw': raw}

  messages = service.users().messages()
  send = messages.send(userId='me', body=body).execute()

Problem: I'm able to successfully send the new messages to my inbox but they are created with a new threadId even though the print statement in the above code shows the same threadId.

Then, when I follow the code example from this thread:

  body = {'message': {'raw': raw, 'threadId': thread_id}}

I get the following error:

googleapiclient.errors.HttpError: <HttpError 400 when requesting https://www.googleapis.com/gmail/v1/users/me/messages/send?alt=json returned "'raw' RFC822 payload message string or uploading message via /upload/* URL required">

Question: How can I generate these emails so they thread with the same threadId?

Note: I have found similar questions here for javascript, and here for java but haven't found an answer for python.