I'm now thinking there should be room for a fourth variant - similar mechanics to the eternal one in how it allows you to choose when to cast, but exhaustible:

Deck of scrolls variant: Draw a card as a half action, it then functions as a scroll of the corresponding spell that only you can cast (casting automatically succeeds). Discard the card once the spell is cast. No further cards drawn will have any effect until that spell is cast. At any point, you may skim through the deck as a full round action to see what spells remain, just as long as you shuffle afterwards.

It has the possibility of a subquest where someone else draws a card from the deck, and until the player induces them to cast the spell it grants, the deck is useless to them.