Also in product-thumbnail.liquid:
find “sizes=” lines and replace first part of them with this (see screenshot):
sizes="(min-width: {{ settings.page_width }}px) {{ settings.page_width | minus: 0 | divided_by: 1 }}px, …
This should fix blurry product images on the product pages themselves too.
Took me forever to find these. But no more blurriness. That’s some janky code there Shopify…