Can’t tell you what the right answer is, but sounds like a decent deal they are offering you. If you take it, remember where you came from and don’t become one of those managers that flips a switch in their head. Take care of your people, have their back, and they will respect you when hard decisions need to be made. Is it a pretty smooth running place, or one that’s riddled with constant drama? Two very different scenarios that would impact you as a manager.
I’d also look into possible job security. Union jobs usually means pretty hard to get let go. Management is normally at-will, meaning if a higher up decides to reorganize they can fire you whenever they please, with or without reason.
Salary, plus OT is not a bad thing. I’ve had salaried positions in my old career and they worked me like a rented mule. Still, even with some extra hours worked you have 700 hours less than your current position so it likely still makes sense. Good luck.