Hi!
Once you use the cards, or by the end of the turn, they are all shuffled back into the deck and you draw 5 cards.
There is no discard deck, just the main deck.
About the space crates, yes you can get rare card drops, and I plan to include more equipment drops, not just on crates.
Thanks for the suggestions, 'life drain' mechanics are not planned for the next cards, but other things are coming. Thanks for the victory screen suggestions.