Selecting an email template and BCC address based on the message type
I would create two new static variables in your class, arrays that contain all options, like

public static $template = [
    self::EMAIL_NEW_SUBSCRIPTION =&gt; DataHelper::XML_PATH_SUBSCRIPTION_EMAIL,
    self::EMAIL_CARD_ADD         =&gt; DataHelper::XML_PATH_NEW_CARD_ADD_EMAIL_BCC,
    // and so on
];


then setting the email template will be as simple as just one line:

$template = self::$template[self::EMAIL_CARD_ADD] ?? '';


and the same goes for BCC