While i agree in most of what you ranted about,
Fed-Imp naval ranks make literally no sense:
you get no navy rank increase from supporting the feds/imps in conflict zones ( combat bonds dont count..)
but being a mailman = admiral/king unless you want to spend a century slowly ranking up...
naval ranks make no sense and a overhaul on it would make the game better!
Whilst I agree with your pain - I'm not sure the logic that you pick is entirely supportive:
The naval ranks for both Fed & Imperial are auxiliary ranks. That is, you aren't actually in the proper Federal Navy - for instance, but in a form of auxiliary, amateur, volunteer support group.
As such, doing menial support jobs 'to help the proper troops' does have an in-story sense to it - however ever unfair. And it is quite clear that both the Feds & the Imps are not that likeable or fair minded. So us saying that this isn't the way auxilliary units work in the US or in Europe is probably irrelevant.
However - I do share your frustration. But is the real reason you even want to progress anything other than to get a Corvette or a Cutter?
Because, if I had my way, I would have both the Federal and Imperial naval rank progression being identical, but stopping before officer level. At that point, progression would be a combination of special missions, and actual option for promotion, and again, that would get you to Admiral in both places.
That should be separate from political position, which - in imperial terms might get you to Emperor or Emporess, and in federal terms might get you to President. In imperial terms, I'd expect you to have to kill yourself into the blood-lines - because historically - if you weren't born into royalty - killing your way in was a legitimate means - a true game of thrones.
In federal terms, you should have to campaign, back-stab, bribe, in an almost identical way - and assassination would definitely be valid. Indeed, apart from different titles, the politics of both seem on a par.
So (i) I agree that naval progression in ED is an illogical grind and needs changing, and (ii) but it could be so much more interesting, and should be anything than just about unlocking a couple of crappy ships...