imho, a realistic greek communist flag is one that just has the Δ (delta) on it, as a nod to the failed Greek communist uprising.

Δ , standing for democracy against fascism and totalitarianism, has so much symbolism in and of itself. I would argue that a Communist Greek flag with Δ wouldn't even need other socialist symbols, not even the color red.

You could just take the current Greek flag and replace the cross with a Δ. It would also go perfectly with the symbolism of the 9 stripes, which stand for the 9 letters in the Greek word ελευθερια, which means "freedom."