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.
Fuck are you on about? There is a really good reason management can't be union members. Your job depends on enforcing the interests of the bosses, if you side with the workers, you get fired and replaced with someone who doesn't. It's literally their job to enforce and maximize exploitation. If someone takes a management position cause it's a way to make a living, I wouldn't call them not a comrade, bi Ut they're on thinner ice. Saying it's necessary to have leftists in leadership positions is silly, the point is to not be led by our bosses or their underlings. If there's already a union, you don't need a manager on your side cause you can instead tell them to take a hike.
Managers are expected to do this when the time comes, but most parts of most management jobs aren't this. Hiring, training, scheduling, resolving (preferably foreseeing) issues, evaluation -- all of this stuff happens in workplaces in AES states, too. Having a manager who at least tries to do those things competently and fairly can be a huge plus for the day-to-day lives of workers.
As for what one does when the bosses call on you to enforce their will? First, you can use your position to frustrate that will, as @CyberSyndicalist@hexbear.net points out. Second, maybe you have to make them fire you at some point -- everyone should have lines they won't cross.
Fully horizontal workplaces can work in some industries, but others (classically, a ship) really do need some people to make decisions and others to follow them. I'd rather look at ideas like democratizing workplaces and electing managers than take the line that managers (even ones chosen by workers) just aren't necessary.
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.