My understanding is: all of this is by design.
Player fleet size limit, late game fleet being stronger than player fleet, officers, ...
The point is precisely to provide a greater challenge compared to previous version of the game, largely preventing player brute force, having player use legitimate tools to succeed (including officers!).
Regarding officer availability, I have not seen any issues: every time I visit a core market in the early to mid game, I look at potential contacts and recruitable officers, easy-peasy. My main complain regarding this process would be the UI, that I don't love. Also, I think I have only used mentoring once, and never used mercenaries.
That said, maybe officer promotion should happen more often, especially very early game? Could happen right at the end of the campaign tutorial - when completing the last mission before leaving starting system, getting some money ensuring player can afford the new officer's pay for long enough. Tutorial would then explain the different method to recruit officers. If there's a guaranteed recruitable officer waiting in Jangala and player is told about it, then player learns by example the other way to obtain an officer while starting his campaign journey.
Also there could be some bar events/missions where player end-up encountering a character that wants to join his fleet as officer (uhhh, this one doesn't already exist, right?).
I remember reading on the forum that some improvements are coming for mercenaries.
By the way, you guys are monsters! Every time this thread gets updated I'm like: this is it, new release available! - Hmmm, wait, what did I just do.