Adding Categories and unique args to emails via wp_mail $headers

I have tried a variety of methods and have been unable to add categories or unique args to emails via the wp_mail function. I want to categorize different emails sent (password resets, etc)

It doesn’t seem to be possible, as it only adds the hard coded “wp_sendgrid_plugin” or categories from the settings page, but just to be sure, What’s the proper format?
Using the API method if that helps. It seems that

$headers[] = “x-smtpapi: “. json_encode(array(‘unique_args’=>array(‘subject_line’=>$title)));
$headers[] = array(…)?
$headers[x-smtpapi] = array(…)?
$headers[x-smtpapi][‘categories’] = array(…)?
$headers[x-smtpapi][‘unique_args’] = array(…)?

if this isn’t possible it should suretly be added, as a filter, with the subject line and to fields as additional args for the filter, and accept the $headers argument in wp_mail.

https://wordpress.org/plugins/sendgrid-email-delivery-simplified/