I found the setting in Customize > WooCommerce > Product Catalog to show both subcategories and products in the catalog. BUT, how do I separate them, i.e., maybe a horizontal rule or order? Right now the first product shows directly after the last subcategory. I’d like there to be separator so the two are distinctly different to the viewer.
It’s not possible using their default HTML structure, unfortunately.
You could try creating a static page, then display the categories and products using their blocks or shortcodes. That way you have control over the HTML.