One of the dumbest things Janeway says is when Harry says that if they were home, he'd be a lieutenant and maybe a LCDR. She says: "you're pretty sure of yourself". Like she actually thinks an officer of his competence should expect to be stuck at the lowest rank for 7 years, when they're not marooned in the Delta Quadrant.
If anything, Harry was being conservative. Consider that in an alternate timeline, he was up for JG in 8 months. If he maintained that pace, he'd probably be at least a LCDR, and I wouldn't even say full CDR is impossible.