Throwing in my two cents about heavy armament:
They can be consumables. (and should take more than 1 unit cargo space per unit)
I'd imagine the danger level of operations is discrete and you can choose to consume some amount of heavy armaments to reduce the danger level of operation.
Of course, making "Lethal" to "Extremely dangerous" would consume much more than making "Moderate resistance" to "Minor resistance". It can/should be some exponentially decreasing consumption per level as danger level go down.
By this context, you're using your property to save your men (by making lower danger level) and make clear difference from bringing ships with the ground support hull mods. (trading ship slot for general maintaining cost)
I think completely negating paying upkeep cost by placing men in storage should be illegal. They should require something like 1/2 or at least 1/5 of their normal cost when standing by in any storage, just like idle admins do. Count the men you send to planet local stash as consumed can easily solve the problem some people talked about (i.e. do not record the amount you put into local stash of any men to prevent people exploiting this to negate paying their upkeep)
People seem to hate the rep loss when raiding and deters them from raiding in early game.
An easy solution is to make rep loss depend on what you've done instead of colony size.
It's just dumb to raid a size 8 when you can raid a much easier size 5 for better loot and much less rep loss.
Marine exp actually sounds awkward to me, as it can be confusing when you're trading them (to Ludds especially lol).
Wouldn't it be more reasonable to use fleet morale instead?
Fleet starts with neutral morale, increases when you win fights with small loss and decreases when you kill a lot of your guys in any way.
The morale would have tendency to go back to neutral as time goes by so it would give incentive for players to chain battles / operations.
Offer the option of fixing negative morale with credit, just like how money can fix colony's unrest.
Using morale would also just get rid of the crew exp problem as the whole fleet shares it.
Morale can also apply to any NPC fleet to fine tune difficulty, for instance, a huge revenging armada should have extremely high morale just to crush you. (And obviously [REDACTED] have no ... well I'm not sure now lol)
The "Morale" system is inspired by the mod
Starship Legends, just apply similar algorithm as a whole fleet