Hiring new marines dilutes the experience and reduces the bonuses (no experience is lost, it’s just redistributed). It’s also possible to drop off experienced marines in storage, or in cryopods, and have them retain their experience level.
that's exactly what a game I'm ripping off shamelessly does, Ultimate General: Civil War! And it worked wonders in that game to make me care about my regiments -- your regiments gain XP over the campaign but they also take losses, you have to manually replenish your regiments in between missions out of your available pool of recruits and the more you fill a regiment with recruits the more you dilute that regiment's overall XP down to 0. You can replenish a regiment with veterans that doesn't dilute XP but unlike recruits they are not free, and bc you need money to buy guns for all regimental replenishment (and upgrades) and bc the AI is actively competing with you over the campaign to defeat you forcing you to go into each mission with $0 in the bank, there's a heavy resource cost to maintaining veteran regiments of any size
that scales with how poorly you did in your last mission, so all the incredibly experienced regiments you have will be small, and all of your green regiments can be as gigantic as your store of guns to equip them, which means that when you get a regiment of veterans up to the max size you
value it, because getting that regiment up to that size has not only required a lot of money, but has required multiple missions of cultivation -- getting them into the battle to get them XP, but also keeping them safe & sacrificing green regiments to defend them from losses.
And because it's rare and hard-won and easily lost once you've got it, the game has the capacity to let you go hog wild once you've gotten them -- in a civil war game of musket-era soldiers you get this regiment of two thousand men, experienced enough to know how to land shots and operate their guns, armed with modern magazine-fed cartridge rifles able to annihilate an enemy regiment from 100%-0 in the space between the start of their charge and when they would have met your men blade to blade if they weren't dead to the man. It's amazing, in the "this is why this method of warfare is dead, you are getting to play the death of a way of life" kind of way few games have every pulled off.
ANYWAY! So my question to Alex is: It sounds rn like you're having marine XP be a global fleet-wide statistic? I think there might be interesting possibilities in having XP tracked per marine stack, that you then assign to objectives in raids
by the stack so that you can have a stack of veterans that you assign to high-priority & low-risk tasks and green stacks of marines that you assign to low-priority & high-risk objectives. This would of course require de-coupling the binary "you require ___ many marines to attempt this objective, if you have that many the objective is automatically won & the difficulty scales casualties" system it sounds like you're going for... maybe, maybe it might not.
Additionally, the moment I make the "per stack XP" suggestion the thought pops into my head of having special missions & objectives in raids that can only be done by stacks of marines of a certain XP level -- right now while it makes sense to have raid effectiveness correlate smoothly to XP it seems a bit weird to have the XP thresholds be purely cosmetic, seems like it wouldn't be v hard to have late-game raiding content gated behind having marine stacks of an adequate size brought up to veteran or elite rank over the course of a campaign