autoptimize_filter_css_defer_inline | WordPress.org

Hi – i like this filter for it’s easier to handle than the rules under the critical css tab. The problem with this filter is, that returning the whole critical css string is rather unwieldy. I tried another approach:

add_filter('autoptimize_filter_css_defer_inline','my_ao_css_defer_inline',10,1);
     function my_ao_css_defer_inline($inlined) {
    if (is_archive() OR is_404()) {
        $inlined = file_get_contents(get_template_directory_uri() . '/css/critical/archive_critical.css');
        return $inlined;
     } else if (is_page()) {
        $inlined = file_get_contents(get_template_directory_uri() . '/css/critical/standard_critical.css');
        return $inlined;
     } else if (is_product()) {
        $inlined = file_get_contents(get_template_directory_uri() . '/css/critical/prod_critical.css');
        return $inlined;
     } 
        return $inlined; 
 }

I used file_get_contents() and this seemed to work, but when i came back to the site i noticed that that the <style> tag in the head was no longer present. I found out that file_get_contents() requires to be echoed and apparently return $inlined does not work.
Do you have a suggestion how to insert the critical css without doing something like: return '<your frontpage CCSS goes here>';?
I’am also uncertain if a condition like is_archive() OR is_404() can be used with this filter.
Thanks for your interest.
theo