// Hide Default WordPress "Add Media" Button for all Non-Admins
add_action('admin_head', 'hbp_hide_default_add_media_button_for_managers');
function hbp_hide_default_add_media_button_for_managers() {
    if (!current_user_can('manage_options')) {
        echo '<style>
            #insert-media-button, 
            .insert-media, 
            .add_media, 
            #wp-content-media-buttons,
            .editor-post-featured-image__toggle { 
                display: none !important; 
            }
        </style>';
    }
}