The expedition mechanic is even WORSE than you think. Stash space/skill points do not stack.

Here is the text from the update that indicates this:

"While Skill Points, extra Stash space, and cosmetics are permanent unlocks, the account buffs will expire if you choose not to depart with the subsequent expedition. Bear in mind, buffs will increase in power for your next three expeditions"

Using reading comprehension here, you can make the conclusion all of the things listed as permanently account unlocks like space, cosmetics and skill points are not something that stack and they said they are permanent and do not go away. They only mentioned temp/account buffs(the resources/repair/xp) stacking and going away.

If you are upgrading and rushing 5 mil thinking you are gonna stack the account unlocks multiple times, you may be disappointed to find out this was a one time deal and next expedition is only higher temp buffs and maybe new cosmetics with no further increase to stash or skill points. Nothing in here indicates those will go up.