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.

mountbatt

This plugin adds a separator between the intro text and the extended (more…) text of the content of a post. It simply replaces the span id more tag with the separator that is set in the plugins php file.
Some separator samples are given there.
Upload into the /plugins directory and activate it in the Plugins Menu.
History:
1.1:
Fixed a bug that forced WordPress Adminmenu to show a white blank page.
1.0:
Initial Release

Add a Separator

Simple Plugin to add a separator between the intro and the (more...) content.

Simple Expand – Lightweight Read more toggle vs Add a Separator