E-mail uploaded file from file field in node to user using Rules + Swiftmailer

I have a node type "invoice" which contains a file field "field_invoice" (pdf). I'm setting up following rules action and am searching for a way to rules pick up the file from "field_invoice" and send out an e-mail with the file attached to an user.

I'm on D9.3.x and have Rules + Swiftmailer running properly.

This is my current Rules

• Event: After saving a new content entity of type Invoice
• Condition: Entity is of bundle (node, node, Invoice)
• Action: Send E-mail

So far I'm good. I can access "field_invoice" in the Send E-mail action.

Next step I tried is creating a custom module with this in mymodule.module:

$file->uri = '<what do I put here?>'; // File path
$file->filename = '<and what do I put here?>'; //File name
$file->filemime = 'application/pdf'; //File mime type
$params['attachments'][] = $file;
$mailManager->mail($module, $key, $to, $langcode, $params, NULL, $send);

function mymodule_mail($key, &$message, $params) {
  switch ($key) {
    case 'my_email_key':
      $message['body'][] = $params['body'];
      $message['subject'] = $params['subject'];
      $message['params']['attachments'] = $params['attachments'];
      break;
  }
}

But I have no idea how to let mymodule pick up my Rule and pass the uploaded file when it runs. There are a lot of similar, well-answered questions, but all dating back from the D7-era with outdated information. I find little documentation or use cases on how to achieve this with D8-D9.

Documentation I found but was insufficient to me:

How do you add a PDF attachment to an email?

https://www.drupal.org/project/swiftmailer/issues/2713651