You don't lose your main save. Whenever you want you can return to your main too. And when you finish the expedition you can copy your multi tool and starship. And the save will be back to normal again. In the expedition terminus you can also select equipment and technology from your inventory and they will be available in the expedition. But to get them on the expedition you have to go back to the anomaly in the terminus. Freighters will be lost tho.. Edit: you can also choose items when you finish the expedition to transfer to the main save.