There's no hook for "give me a post ID before the post has been created".  You only get an ID after the post has been saved.  So yes, use the hook `'content_save_pre'` to modify your content before saving, then once you get back a post ID use that ID to update the post featured image.  

There's a great answer describing how to acquire and set the image for a post, [here][1]

And, the hook you'd use to trigger that set image routine would be `'save_post'` which is documented [here][2]


  [1]: http://wordpress.stackexchange.com/questions/40301/how-do-i-set-a-featured-image-thumbnail-by-image-url-when-using-wp-insert-post
  [2]: https://codex.wordpress.org/Plugin_API/Action_Reference/save_post