To put this into perspective, I hosted a CRPG for not that long and my user data folder has 430 individual entries, one for each person. On a smaller project, being generous, let's say that for each player there is the need to store 6 entries about them, that is 2580. As this project sounds, there needs to be a lot of data stored. Just brainstorming and guessing I would assume that there is need to save the player's financial data, spaceship name, city name, city progress, level, fund income, if one would be making something like a strategy game with separate buildings to manage, you'd also have to keep track of those, if all that would be true then one can safely assume each player needs at least 15 entries if the game goes in-depth enough, that's if there would be no individual skill progression. So 15 entries for each player, for 430 players would take 6450 variables to load at startup.
Keep in mind, 430 players is a low number, my server didn't run for very long and CRPG-s are not appealing to many people while an original concept like this certainly would be.
Now as to actually help the OP, here is how I am handling loading and saving data. Do NOT use this, it is a WIP, I just copied it straight from my very WIP mod, that and this is custom-made to fit the mod I am making and would do well only in a few scenarios I can think off from the top of my head.
PastebinUnrelated side-note, if someone can advise me how to remake the item saving-loading, let me know via steam.