PHP "rename image" function half working

Hi,

I'm trying to install a hook to rename images and fill the alt field automatically based on Post Title.

I found this code :

function file_renamer( $filename ) {
    $info = pathinfo( $filename );
    $ext  = empty( $info['extension'] ) ? '' : '.' . $info['extension'];
    $name = basename( $filename, $ext );

    if( $post_id = array_key_exists("post_id", $_POST) ? $_POST["post_id"] : null) {
        if($post = get_post($post_id)) {
            return $post->post_title . $ext;
        }
    }
    
    $my_image_title = $post;
    

    $file['name'] = $my_image_title  . - uniqid() . $ext; // uniqid method
    // $file['name'] = md5($name) . $ext; // md5 method
    // $file['name'] = base64_encode($name) . $ext; // base64 method

    return $filename;

  }
add_filter( 'sanitize_file_name', 'file_renamer', 10, 1 );

/* Automatically set the image Title, Alt-Text, Caption & Description upon upload
--------------------------------------------------------------------------------------*/
add_action( 'add_attachment', 'my_set_image_meta_upon_image_upload' );
function my_set_image_meta_upon_image_upload( $post_ID ) {

    // Check if uploaded file is an image, else do nothing

    if ( wp_attachment_is_image( $post_ID ) ) {
        
       // Get the parent post ID, if there is one

        if( isset($_REQUEST['post_id']) ) {
          $post_id = $_REQUEST['post_id'];
          } else {
          $post_id = false;
          }

            if ($post_id != false) {
        $my_image_title = get_the_title($post_id);
            } else {
        $my_image_title = get_post( $post_ID )->post_title;
            }
        // Sanitize the title:  remove hyphens, underscores & extra spaces:
        $my_image_title = preg_replace( '%\s*[-_\s]+\s*%', ' ',  $my_image_title );

        // Sanitize the title:  capitalize first letter of every word (other letters lower case):
        $my_image_title = ucwords( strtolower( $my_image_title ) );

        // Create an array with the image meta (Title, Caption, Description) to be updated
        // Note:  comment out the Excerpt/Caption or Content/Description lines if not needed
        $my_image_meta = array(
            'ID'        => $post_ID,            // Specify the image (ID) to be updated
            'post_title'    => $my_image_title,     // Set image Title to sanitized title
            'post_excerpt'  => $my_image_title,     // Set image Caption (Excerpt) to sanitized title
            'post_content'  => $my_image_title,     // Set image Description (Content) to sanitized title
        );

        // Set the image Alt-Text
        update_post_meta( $post_ID, '_wp_attachment_image_alt', $my_image_title );

        // Set the image meta (e.g. Title, Excerpt, Content)
        wp_update_post( $my_image_meta );

    } 
}

I installed it in my function.php and it's half working.

It's just working on uploading images but not with existing image attached to post.

I may be understood bad, but I thought this function would to both : rename on upload AND rename existing images.

Do you think I miss something here ?