Any way, hook to add content right before the “read more” link?

Yes, you could use the filter the_content_more_link.

Here is the example

add_filter( 'the_content_more_link', 'q363666_tweak_more_link', 10, 2 );
function q363666_tweak_more_link( $more_link, $more_link_text ) {
  // WordPress prepared the $more_link
  // you can intercept here and return whatever you need
  return 'Hello World ' . $more_link;
}