Jonas Hjalmarsson

A single shortcode you can drop into any post, page, or text widget to get a click-to-expand “read more” toggle. The output is a native HTML <code><details></code> element styled with a small inline stylesheet — nothing else.
The point is the absence of features. There is no settings page, no animation library, no editor block panel with twelve toggles. If you need a read-more on one paragraph in one post, you do not want a plugin that wants to manage your whole site.
Usage:
<pre><code>[expand title="My title"]
The content goes here.
<img src="even-images-can-go-in-here.jpg" />
And wrap up with a closing expand tag.
[/expand]
</code></pre>
Supports a custom CSS class via <code>[expand class="my-class"]</code> and works with WordPress oEmbeds and other shortcodes nested inside.

Simple Expand – Lightweight Read more toggle

Adds an [expand] shortcode that wraps content in a native HTML `` toggle. No JavaScript, no settings page, no bloat.

ramon fincken

Auto sets the more tag to post title if no custom text has been set. 
 Auto text is customizable using mp_auto_more_more_link_readmore filter in the Premium version: 
<a href="https://webshop.mijnpress.nl/shop/wordpress-auto-more-tag-text" rel="nofollow ugc">Buy premium</a>
Coding by: <a href="https://www.mijnpress.nl" rel="nofollow ugc">MijnPress.nl</a> <a href="https://twitter.com/#!/ramonfincken" rel="nofollow ugc">Twitter profile</a> <a href="https://profiles.wordpress.org/ramon-fincken/" rel="nofollow ugc">More plugins</a>

MP Auto more tag

Auto sets the more tag to post title if no custom text has been set.

Simple Expand – Lightweight Read more toggle vs MP Auto more tag