I thought of a good way to have way more detailed crew without having to many!
Crew stats! so basically the same crew, green, regular, vet, elite but they would have 3 values: combat, flying, repairing. Marines would literally be crewmen with good combat stats but they would also help run your ships and during combat the highest stats would be chosen first. You can then attach themes after the rank so 'green junk pirate crewman: combat 3/20, flying 4/20, repairing 6/20'. Once the crewmen past a certain skill threshhold you can then change it to 'regular junk pirate mechanic: combat 5/20, flying 6/20, repair 10/20' it would add a massive dynamic to your crew so they arent just ammo anymore and you would feel the loss of each and every man.
If you fixed the stats for each rank and theme you'd split the stack of 500 green crewmen into 3-4 stacks of different green crewmen and you could specialize the hell out of your crew. Going on a hopefully salvage only mission? bring along more mechanics and marines then flight deck crew! going on a fast raid? make sure those mechanics are good fly deck crewmen to squeeze more out of your ships!
You would also add so much more flavor to the different factions and planets, instead of just a random name and number you now know that corvus II produces excellent line crewmen who excel at flying and are good in a boarding action but they lack the mechanical talent of corvus IV with its junk pirate crewmen. You'll return back to that one system that produces excellent rounded crewmen instead of just buying the same crewmen from any station you happen to be by.