Actually, you can disable the feature image through the theme by default for single posts and pages separately. You don’t need to any additional third party plugin for it. To disable the feature image from single post, please go to Dashboard > Appearance > Customize > Theme Options > Single Post Page Options. Then click on the Disable Feature Image option.
To add the feature image in the content. we would like to recommend to disable the feature image from above option and add the image (which you want to add on feature image) directly in the editor. Being compatible with new block base editor Gutenberg, you can make your contents in any layout.