The memory half
Vocabulary is the canonical spaced repetition case and there is no cleverer approach: the word means what it means, there is no mechanism to derive it from, and you simply have to remember it. Cards, scheduled properly, and the problem is genuinely solved.
- Card in context, not in isolation. A word on its own is a translation; a word in a sentence is usable language.
- Both directions, but weight recognition first. Recall (English → target) is much harder and much more valuable — do it, and expect it to hurt.
- Mine your own sentences from what you're actually reading. Somebody else's frequency deck is somebody else's life.
- Add audio. A word you can read and cannot hear is a word you don't have.
The skill half, which cards cannot touch
Listening, speaking, and the automatic deployment of grammar are skills, and skills are built by doing the thing badly for a long time. There is no shortcut and there is no deck. The student with five thousand cards and no conversation practice has optimised one half of the problem to the total exclusion of the other, and it is the more common failure by a wide margin — because cards are comfortable and speaking badly to a stranger is not.
Grammar in particular is worth understanding once, properly, and then acquiring through exposure. Drilling grammar rules as facts produces someone who can conjugate on paper and freezes in speech.