How dynamically change wp_mail behaviour, sending html or plain text based on conditions?

Question

Here’s an (untested) PHPMailer example to check for e.g. the subject and the content type:

function mailer_config( PHPMailer $mailer ) {
    if( 'Subject #1' === $mailer->Subject && 'text/html' !== $mailer->ContentType ) {
        $mailer->IsHTML( true );
    }
}

other options would be to e.g. check the $mailer->From or $mailer->FromName, or some other conditions, depending on your setup.

Another approach without PHPMailer dependency, could be to use the wp_mail filter, with the wp_mail_content_type filter.