So through a variety of work-related drama that really doesn't matter, my direct manager's position has opened up. I've essentially been doing the job but without the title for a fair bit of time so I decided I might as well try to get the position officially. I later found out that the position I applied for isn't actually a union position and it's kinda made me start rethinking things, as far as I'm aware it shouldn't actually affect the benefits or hours, just a non-union unit manager position.
Who benefits from OP staying put? This is a huge meatball right across the plate for everyone in the union to have a sleeper cell in an actual management position. If OP doesn't do it, the union going to get more of the same. If they hire someone from outside the department or company, it will almost certainly be much worse for everyone except upper management.
Also, don't be quick to assume OP would be found out so quickly by the rest of management, I've been doing this for years now - upper management thinks I'm great and one of their buddies, meanwhile I let my team take random days off without using up PTO, arrive late and leave early without question, etc. The work still gets done and no one gives it a second thought. If you're a professional bullshitter and/or your own boss isn't a micromanager, it's very possible to make a lot of good people's lives much less miserable by going this route.