Start on a phone, on a real SKU, with a real variant
Open a best seller, an out-of-stock SKU and a variant-heavy product. Desktop screenshots lie. Shopify themes often look finished on a 1440px canvas and fail when the buy box drops below a 900px gallery.
A product page audit should sample, not crawl 50k handles on pass one. You need enough SKUs to see how the theme treats sold-out, subscriptions and option-heavy catalogs.