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I would add that since you are "head of the department" I feel that your bosses trust you enough to run with you if you just tell them that what the department need is a full-time manager.

You are also in a very privileged position of defining the role of that manager. You like some managing tasks? You can keep them. You can even make it your hire, and hire the kind of manager you want to help you with these tasks. Since you can recruit yourself, that person will have some loyalty toward you.

I think you are in a very good position to build your dream job from scratch. Just do it my dude :)