After that the HR manager now has the option to notify all the other Head of Departments (A role) (Multiple people) about this new job opening except the Head of Department related to this job opening.
I think you should use the notification because a notification has an option of condition to mail sent and also you can manually set the Head of Departments mail-in Recipients table.
When you post a job opening and approve it by the manager then according to the workflow stage define condition in condition section.
//eg.
doc.status == "Open"
OR
doc.workflow_state == "Approved"