Changing library path with hook_library_info_alter
I pushed my patch for social icons module to add dynamic font awesome css here
https://www.drupal.org/project/social_icons/issues/2984954

This is helpful to you.

if (isset($libraries['font-awesome'])) {
     $library_key = key($libraries['font-awesome']['css']['theme']);
     $library_array = $libraries['font-awesome']['css']['theme'][$library_key];
     unset($libraries['font-awesome']['css']['theme'][$library_key]);
     $libraries['font-awesome']['css']['theme'][$font_awesome_url] = $library_array;
}