"-t" mode not supported and also BCC not working

The code you link to is from Symfony mailer library - it's not part of this module. So there's nothing we can do about it here you need to raise an issue with the library.

Have you reviewed the comment from Symfony\Component\Mailer\Transport\SendmailTransport? It states:

* Supported modes are -bs and -t, with any additional flags desired.
*
* The recommended mode is "-bs" since it is interactive and failure notifications are hence possible.
* Note that the -t mode does not support error reporting and does not support Bcc properly (the Bcc headers are not removed).
*
* If using -t mode, you are strongly advised to include -oi or -i in the flags (like /usr/sbin/sendmail -oi -t)
*
* -f flag will be appended automatically if one is not present.

In light of this information, particularly the recommendation for using the "-bs" mode for its interactivity and failure notification capability, you might consider revising the $settings['mailer_sendmail_commands'] in the Drupal settings configuration. If you prefer to use the "-t" mode despite its limitations in error reporting and handling of Bcc, you should include the '-oi' or '-i' flags for better operation. Thus, the settings would be modified to:

$settings['mailer_sendmail_commands'] = [
  '/usr/sbin/sendmail -t -i',
];