The following might work for you.
- Create a series-like column "#universe", "Reading Order".
- Fill in the new column as you wish. The series name is the universe, the series index is the order to read the universe.
- Search "#universe:true"
- Select all
- Create a CSV catalog with #universe, #universe_index, and any other metadata you wish. Put the columns in the order you want them in the eventual EPUB.
- Open the .csv in LibreOffice (what I have)
- Sort by #universe, #universe_index, whatever
- Save as HTML
- Import the HTML as a book in Calibre.
- Convert to EPUB.
This produces a simple EPUB with the books in reading list order. Example using test data: