If I were in your shoes, the very first thing I'd do is set up two different post-purchase surveys: one for new customers and a separate one for returning customers. It seems small, but this split is the most critical piece for understanding why people come back. You have to ask different questions to people at different points in their journey with you.
My thinking here is really shaped by people like Matt Bahr, who pointed out on The Unofficial Shopify Podcast that asking a fifth-time buyer