... Harry didn't have a staff. His 'department' was his console connected to the ship's sensors. If there's a problem with those sensors, it's Belanna's territory, if there's an intruder near a sensor, it's Tuvok's problem and book-smart Harry is a nobody next to Seven when it comes to upgrading the ship. Harry's rank was fine with his position.
Even if we're just talking about his bridge duties, the crew who manned his station while he was sleeping, while he was on away missions, in meetings with Janeway, while he was on the holodeck getting chumped by girls, during his off time... That's his staff, and considering how much time he didn't spend sitting at the operations station on the bridge, I suspect that it's at least 5 people...
The other operators sound more like coworkers than underlings. If there was a problem while Harry was off duty, they'd report to whoever was in command, not Harry, and when he comes back on shift, he won't need a briefing or a PADD because all the information is on his console. On the other hand, if there was a catastrophic problem in engineering, dangerous space to traverse, or tactical issues to resolve and B'ellana, Tom, and Tuvok were off duty, they'd be called in.
With Star Trek, seniority doesn't automatically mean you get promoted. In that TNG episode were Q "saves" Picard from being stabbed during college, he wasn't the Captain he was ultimately supposed to become because he didn't take risks. That episode shows that regardless of how long you serve, promotions goes to those willing to risk it all. Tuvok's getting shot at, Tom's dodging shit, and B'ellana's stopping the ship from blowing up (and covering Harry's butt should any of his systems fail). What risks does Harry take upon himself?