If you cast a card that swaps position from Astral Heron like Resonating Strike to move him to a battlefield, do you get his trigger as the first spell would be Resonating Strike?

Yes. The "if I'm at a battlefield" check happens after your first card fully resolves (Rule 383.2.c), and Resonating Strike's move happens during that resolution — so when the condition is evaluated, Heron is already at the battlefield and the trigger fires. Your next card that turn costs [2][A][A] less.

This is the mirror of the Retreat case (FAQ #12025): moving Heron off the board before the check fails the trigger; moving him onto a battlefield as part of the first card's resolution satisfies it. Note Resonating Strike being a [Reaction] spell doesn't matter — it's still your "first card each turn" regardless of when you play it.