hey so, in the later stages or just after the first couple, unlocking more slots should be tied to mastery of certain skills. it can either be pure mastery of that skill or an unlock to buy like you have now.
Around sector 5, I'm able to go on a min-unlock spending spree on all the cheaper skills and collect all 51 by sector 7. That might have something to do with having prestiged 7 times.
On that note, prestige giving so much xp boosts via repeat-prestige def shortens the lifespan of the game. I normally hate nerfs, but prestige needs a major nerf. Maybe instead of current, it allows a +1 max tier on a module if that module is collected again - and the next time you prestige you can select a different module. Like say choosing the global cooldown module first and then the evasion module.
On the note of the evasion module, the evasion skills is so broken. Coupled with the evasion module, I get hit like 1 out of 20 times. With everything else I have stacked on top of that, I feel like the villain.