Language learning is a long, long process, and it’s important to make sure your habits are sustainable. It doesn’t really matter what’s optimal if you get demotivated and stop learning, so above all, you should do whatever keeps up your learning process. Don’t force yourself to speak the flashcards aloud if that will discourage you from the whole thing.
That, and don’t worry about optimal. There are no bad habits that can’t be unlearned (and the value you’d get out of speaking would far outweigh any effort you need to invest in the future if you want to improve your accent). Speaking would be great, but as long as you’re learning grammar and vocabulary, you’re on track.