Definitely a tough one.
If you Google “WordPress first image as featured image” you’ll find some options.
This is one of them: https://wordpress.org/plugins/get-first-image-set-as-featured-image/
However, that’s a lot of posts, so I would definitely run some tests on a dev site before committing to the big site. I would also take full backups.
Then you can remove the featured image from the single post in “Customize > Layout > Page Header”.
Hope this helps 🙂