Spaced repetition software is perhaps the most effective way to study using flashcards. The idea is that we review new material right before we forget it, then we wait longer before reviewing it again.
Lather, rinse, repeat.
Each review, strengthens the memory.
Language, though, is learned through interaction with material that is at least ~95% comprehensible, rather than like a collection of facts to memorize. This can happen via reading, listening, conversations etc.
We need a lot of input for the brain to acquire language.
It's simply how the brain works.
For Greek, this means lots of reading (it is a classical language after all), but reading at or near one's current level.
Repeated exposure to comprehensible input, allows out brain to acquire the words and grammar in that input naturally.
But we need repeated exposure.
Could we perhaps combine spaced repetition with reading of comprehensible input to get the needed repetition?
How to combine them
Here's how I've done this.
I have two decks. One for shorter sentences that have words that I specifically want to learn. A second deck that just has the "address" of what I want to read.
In the first deck, I make a card with the sentence I want to review and bold the words I want to learn. Then I put the meanings of the new words on the back of the card.
For the second deck, I make a card that tells me what to read. I don't put anything else on these cards. For example, I have some cards that say
I Clem 17 (1st Clement, chapter 17) or
Rom 6 or
I Clem 20:1-3.
The second deck takes much more time to review and I need access to the books I'm reading. The first deck I can study on the go or whenever I have a few minutes.
For the second deck, I'd recommend creating cards with smaller chunks of text to read. So instead of a card that says
I John 1, you might consider
I John 1:1-4 and then making other cards for the rest of the chapter.
This means that you don't need as much time to reread the material and are more likely to keep at it.
Of course, you don't need a spaced repetition app to do this. The idea is simply to read and reread the same texts. This let's you learn grammar and vocab in a natural way.
The trouble is finding texts to read that are level appropriate, but that is a can of worms for another post ... or Ph.D. dissertation ;-).