@Rapha The main point of the ellipsis is to create uniform CMS items by setting a max height for text content while also cutting off the text in a elegant way. If there was an option to extend the text then it would affect the other CMS Items by extending their height as well and we would be back to square one.
However I think you are trying to create this feature for individual CMS page where extending the text content wouldn’t break immersion. My solution would be to duplicate the rich text content and have the full content hidden. Create a button and add an interaction when someone clicks on it, the truncated text will become hidden and the full text will be displayed and vice versa when someone clicks again.