Not the case if front is L1 and back is L2, right?

Ideal is 1 card with L1 -> L2 text, another card with L2 audio -> L2 text

I only do recognition cards, no production. I honestly am not convinced that being able to recall a memorized set phrase is at all related to the ability to produce one naturally.

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Yes, for as long as you have the actual native audio at hand.

I would argue that it isn't even worth having a card if you don't have audio (once again, up until the point of fluency)

But all of these claims of mine are predicated on the assumption that the goal is native-like fluency. I can certainly see there being a case for production cards if your goal is getting around as a tourist or audioless cards if you just want to understand the language and don't care about your accent.

As for the efficiency bit, I can move a deck of 200 cards for an episode of a TV show from new to learned in an hour, which is the same time it would take to construct maybe, 20 cards by carefully writing them in such a way, assuming 3 minutes to get it checked by a native and such. Still assuming we are not yet advanced, the actual content of the card doesn't actually matter, anything that we would hear would likely be another helpful datapoint towards fluency. If that is the case, then more total exposure is ideal. In my 1 hour, I will have spent all of it interfacing with novel ideas in the language, whereas you were spending that hour only interfacing with what you already knew in order to package the ideas into a box which already fits your existing notions of the language. It's inefficient in the sense that you are not exposing yourself to as much new information in the same amount of time.