If you play Astral Heron to a battlefield as your first card, does it see itself and turn the cost reduction on the next card?

Yes — one card. You play Astral Heron as your first card of the turn. Once it's finalized on the battlefield, its triggered ability checks the condition. Since it sees that you've played your first card and Astral Heron is now at a battlefield, the trigger fires and gives the discount to your next card.