Don’t know if you’ve thought about this, but this is a perfect candidate for a custom post type and custom taxonomies. This is a good plugin for creating them: https://wordpress.org/plugins/custom-post-type-ui/
Then you can have one or more custom taxonomies for your categories and it becomes quite a bit simpler to maintain than putting this in either pages or posts. Custom post types and taxonomies can also have their own templates that are automatically used by WP, so then there is no need to the admin to select a special page template to use for a category or a product.
