Question about filter / action priority

in my main plugin I filter the_content to add rich snippet, something like this:

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in my main plugin I filter the_content to add rich snippet, something like this:

add_filter('the_content', 'add_schema');

function add_schema($content) {
        $schema=NULL; //To avoid undefined variable notice outside the loop

        $review_choosen = get_snippet_type();

        //Use apply filter to allow users to customize the rich snippet
        $schema = apply_filters( 'filter_schema_microdata', $review_choosen )

        //If $schema is not false mean the user added his filter and return this one
        if ($schema) {

            return $content . $schema;

        }

       //DEFAULT CODE HERE

}

At the begin of the function, I use apply_filter to let my user or other developer to customize the rich snippet instead use the defaults that my plugin adds.

If the add_filter is put on functions.php, everything is ok.

But now I’m working on an extension that run on plugins_loaded. So when I try to use add_filter here this is what happens:

If I use

add_filter ( 'filter_schema_microdata', 'callback_function' )
    function callback_function ($review_choosen) {

the $review_choosen var is empty.

If instead, I try to put an higher priority, e.g. 9

add_filter ( 'filter_schema_microdata', 'callback_function', 9 )
   function callback_function ($review_choosen) {

The $review_choosen var is ok, but the functions doesn’t return nothing.

So in which action should I run the add_filter?