I took the plunge and switched from swiftmailer to symfony_mailer in StratosERP on the weekend, and it was surprisingly simple!

This snippet of code generates a pdf file using entity_print with wkhtmltopdf in the private filesystem, creates a new email, attaches the file and also takes a text_format field for the body and sends it.

// Create the Print engine plugin.
$printEngine = $this->printEngine->createSelectedInstance('pdf');

$filename = 'output_filename.pdf';

$uri = $this->printBuilder->savePrintable([$entity], $printEngine, 'private', $filename);

$email = $this->emailFactory->newTypedEmail('se_email', $entity->bundle())
  ->setSubject('Test email with attachment')
    '#type' => 'processed_text',
    '#text' => $email_text['value'],
    '#format' => $email_text['format'],

$result = $email->send();