When an article contains only an extract of the content, you could fetch the original web page and apply a set of rules to get relevant contents.
Miniflux uses CSS selectors for custom rules. These custom rules can be saved in the feed properties (Select a feed and click on edit).
div#articleBody: Fetch a div element with the ID articleBody
div.content: Fetch all div elements with the class content
article, div.article: Use a comma to define multiple rules
Miniflux includes a list of predefined rules for popular websites. You could contribute to the project to keep them up to date.
Under the hood, Miniflux uses the library Goquery.