I love the options you’ve build for blog featured images. I’d like to replace the content you have in your demo but it looks like I can only choose a single image at a time (attachment).
I hope you are well today and thank you for your questions.
The WordPress 5.0 came with Gutenberg editor as a default editor so to use those options, you have to switch from Gutenberg editor to old editor using classic editor as described in the below pages.