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I agree with effect and Joe
Even tough search engines don't understand the actual content (as far as we can tell :P). As a rule of thumb you can always ask yourself the question if your content adds useful information for your visitor. Subcategories contain extra information about the products and can help visitors find the product they are looking for faster. So I would definitely keep them in.
Most Wordpress AMP plug-ins put your content in a lay-out that is fully AMP compatible. This means that you are good to go from a SEO perspective. The downside is that these plug-ins don't care about the design of your website. If you are looking for a more immersive environment it's good to look for an AMP enabled theme that contains AMP pages that are in the same or similar style as the regular website.