Hello There,
Thanks for writing in! By default, the featured image is an image element when you view the single blog post. In the blog index, once the post item is assigned as featured post, the post item will display a different layout displaying the featured image as a background for the post title.
With your request, " to have my featured image have full width so that it covers the entire screen. On top of the featured image, I’d like to have the post title.", Regretfully, this particular customization is outside the scope of our support as this is not related to an issue with the theme and instead has to do with your customization of it.
Please note that issues that might arise from the use of custom modifications and further enhancements should be directed to a third party developer.
Thank you for your understanding.