Need Help With High LCP On Mobile

Dear Team

I use GP premium, WP Rocket, and Shortpixel.

Made sure everything is alright from the other two plugin's end (I'm creating webp images and serving the same using WP Rocket).

As per your suggestions in other forums, I've tried not to lazy load my feature image using this PHP (and adding first-feature-image in WP Rocket)

// Add skip-lazy class ( or any class ) to featured image of latest post
function skip_lazy_class_first_featured_image($attr) {
    global $wp_query;
    if ( !is_single() && 0 == $wp_query->current_post ) {
        $attr['class'] .= ' first-featured-image';  
    }
    return $attr;
}
add_filter('wp_get_attachment_image_attributes', 'skip_lazy_class_first_featured_image' );

And preloading the featured image using this code -

add_action( 'wp_head', function(){
    $id = get_post_thumbnail_id();

    if ( is_single() && $id ) {
        $size = 'full';
        $img_src = wp_get_attachment_image_url( $id, $size );
        $img_srcset = wp_get_attachment_image_srcset( $id, $size );
        echo '<link rel="preload" as="image" href="' . $img_src . '" imagesrcset="' . esc_attr( $img_srcset ) .'" imagesizes="100vw">';
    }
} );

Can you guide me how I can reduce the LCP on mobile? I'm using a good server and even cloudflare Argo for delivery. There must be something wrong with the delivery, image size or something I can't infer right now. Can you help?