Nobody wants to be a middle manager. I’d be concerned if anyone ever wanted to be a middle manager. If you’re not C level and somebody reports to you, you’re a middle manager though.
You’re a middle manager if you’re not in “upper management”, but managers report to you. It’s called “middle” management because it’s between upper management (C-level and VPs, probably) and first-line management / frontline management. Start with all managers, select the ones in the middle, that’s middle management.