Optimal unlocking mechanism for booster cards is unclear

When creating booster cards, there are a few different mechanisms for unlocking the cards that one can imagine:

• time-based unlocking: 3 months after you read the initial essay, you go to collect booster cards, or the booster cards get added to your deck in one or more installments.
• content-based unlocking: when you complete chapter 5 in a book, you go to a special place to collect the booster cards for chapter 2 (or maybe they just get added to your deck automatically). One reason to prefer content-based unlocking (or some combination of unlocking mechanisms that includes content-based unlocking) is that it helps with memory interference: if you learn some related-but-distinct content and then come back to the original topic, you will have some to deal with interference from the newer content. This helps solidify both the old and new topic because you're forced to discriminate which is which.
• review-based unlocking: each time you go for a review, a small amount of booster cards get mixed in with material you've already seen. I've been doing this manually and informally with Dawnguide: each time I get an email reminder to review my existing cards, I read a new article (which adds a few new cards about a different-but-related topic; I don't literally get booster cards, since there are a limited number of cards for each article and I collect all of those on my first reading).

actually there's two axes here: how the unlocking is paced, and whether you go out of your way to collect the cards (possibly reading some review material or "booster material" as well) or whether they get magically added to your deck (in this latter case, there is the further question of whether to make booster cards visually distinct from existing cards).

With Tao Analysis Flashcards I am aiming to do content-based unlocking because there is no other way to incorporate booster cards in Orbit.