They've set it up so that you arrive at the endgame far too early (any real reason that you need to level up after every 3rd mission or so?),
That's entirely driven by the amount of content they have. As they expand mid-level content, the XP for the existing content will be nerfed to maintain progression.
The problem they run into is they're a small team, so any time spend working on mid-level content is time not spend on endgame or all-level content. More importantly, time spent on the F2P move and the codebase resync was time that couldn't be spent on *ANY* episodic content.
One of those major efforts is now complete. The other is approaching completion. They're increasing staffing in the content areas. These things will combine to allow them to slow leveling by adding content in the areas where you blow through currently.
Of course, that matters more to NEW players than existing ones, and they will never slow leveling at the lowest end because that's the least-interesting part of a characters' life. Most of the playerbase is in the Commander/Captain range, so that's where they'll have to concentrate this.
Almost all of us reading this (or at least the ones who comment) are at the high end. None of this will really affect us on our mains. That's a hard balance they have to walk; where's the most effective place to spend a developer's time?
A good story mission takes several people a week to create, is only seen by people in a narrow level range, and lasts those people 15-60 minutes.