I was able to do this with a couple lines of CSS. The only part I don’t like is after you click read more the rest of the text will start on the next line. That’s part of the reason I added the … at the end.
.inline-show {display: inline;}
.show_more {display: inline-block;}
Right before the shortcode put some text in a p tag with class inline-show.
<p class="inline-show">This is text...</p>