{$contribution_aggregate} not outputting in message template
Look in your civicrm.settings.php (in wp-content/uploads/civicrm). There's a line that says:

define( 'CIVICRM_MAIL_SMARTY', 0 );

Change it to

define( 'CIVICRM_MAIL_SMARTY', 1 );

See https://docs.civicrm.org/user/en/latest/common-workflows/tokens-and-mail-merge/#smarty