heres a suggestion for re-implementing old crew mechanics in a way that's less esoteric than it was;
there are two kinds of crew items, Crew & Veterans. The "Crew Training" skill is offset onto the veteran item; ships get increased CR & decreased supply usage for the amount of its crew requirement provided by veterans, up to 15% & 20%, and when a ship is 100% crewed by veterans it also gets a +30 seconds peak operating time and prevents ship-loss accidents (effectively giving you at least 1 free accident before its possible to lose a ship from being out of supplies). After combat, a proportion of your XP gained causes crew to transform into veterans. The "Crew Training" skill increases the rate of crew conversion, its elite tier makes a proportion of all crew purchased become veterans and makes that officer's flagship get the accident-preventing bonus from having any amount of veterans (allowing your flagship to run on 0 supplies until you run completely out veterans. Some Snow-Piercer maintenance. Get in the reactor, shinji!).
On the fleet screen you can assign ships 3 tiers of priority; 0 is default (all spare crew & veterans are split between 0's), 1 pulls crew from the 0 ships until it's full if there's not enough to go around, 2 pulls veterans from 1 & 0 to have as much CR as possible & crew also if the fleet has a manpower shortage. The drawback to prioritizing a ship is that when it takes damage you're losing veterans instead of crew. if you know a ship is going to die a lot (like if ur running a hulk d-mod fleet..) you can benefit from setting your hulk ships to 0 or 1 priority to save your veterans, and bc veterans reduce maintenance use you're also incentivized to prioritize some non-combat ships that have particularly high maintenance
Additionally, there's no way to produce veterans from markets. They're crew hardened in battle, the only thing that produces them is fleets standing down from service at a market, and they get consumed by fleets prepping for duty at a market so they are effectively impossible to find in any quantity. The only way to get them is to train them in battle, and you get them at a rate so low that damage taken in battle doesn't just cost you supply it also depletes your fleet of veterans unless you use skills & hullmods that reduce crew loss on your high-priority ships to plug the gaps enough to give you a positive veteran intake, which connects the new crew skill mechanic to character skill developement, officer management AND ship customization.