I do not want to depend on a plugin for this – too unreliable. I do have one though: the dev site is Organize Series, to the WP plugin Directory. If a plugin is abandoned I would have to find/replace/set-up each series with a new plugin. Sigh.
This article suggests creating your own plugin. But how do I update it? The article is ancient. 2012. https://code.tutsplus.com/tutorials/adding-post-series-functionality-to-wordpress-with-taxonomies–wp-25414