The reason the excerpt shows is because loop-index.php is the template for the Blog page as well as the individual posts. Now ordinarily we'd suggest you use the <...more...> but since you said you had a lot of posts we'll try this.
1) Save a copy of loop-index.php as loop-single.php in the same includes folder.
2) Now open loop-single.php and the "the_excerpt" line you added to this:
the_content(__('Read more →','avia_framework'));
3) Now open single.php in the theme's root directory and change this line
get_template_part( 'includes/loop', 'index' );
get_template_part( 'includes/loop', 'single' );
What that should do is leave the Blog's main page as excerpts and the single pages as full posts. Mind you any future updates will override this customization.
Hope this helps!