Okay, so I think I’ve figured out how to get my categories and tags index pages to display the titles and subtitles to my liking. It is possible to do this by looping through all the site’s pages. There, you will have access to each page’s front matter variables, which you can then use to render your list items with titles and subtitles. The nifty trick of including a page to get it to render using a “layout” that specifies merely a list item, well I couldn’t get that to work due to the front matter getting included verbatim rather than being parsed out.
For larger sites, you are recommended to use collections so that you can narrow down looping to particular namespaces.
20180601/DuckDuckGo jekyll include page front matter
20180601/https://stackoverflow.com/questions/50246063/include-front-matter-from-another-md-file
20180601/https://jekyllrb.com/docs/datafiles/
20180601/https://jekyllrb.com/tutorials/navigation/#scenario-8-retrieving-items-based-on-front-matter-properties
20180601/https://jekyllrb.com/docs/collections/
20180601/https://jekyllrb.com/docs/variables/