Read time as post meta data

I'm using GPT's help to try to add a function to store reading time as post meta data and then pull it using dynamic content from Generate Blocks. I want to display it in the post content template (block) and blog content template (block) I created.

But when I call the metakey reading_time in dynamic content of GB nothing is displayed. In the preview of the block - content template for blog archives I see it displays 0 (which is not right either).

This is the code:

function read_time() {
    $content = get_post_field( 'post_content', $post->ID );
    $word_count = str_word_count( strip_tags( $content ) );
    $read_time = ceil( $word_count / 240 );
    return $read_time;
}

function add_reading_time_meta_key() {
    register_post_meta( 'post', 'reading_time', array(
        'type' => 'number',
        'description' => 'Estimated reading time in minutes',
        'single' => true,
        'show_in_rest' => true,
    ) );
}
add_action( 'init', 'add_reading_time_meta_key' );

function update_reading_time_meta_key( $post_id ) {
    if ( ! isset( $_POST['post_type'] ) || 'page' === $_POST['post_type'] ) {
        return;
    }
    $read_time = read_time();
    update_post_meta( $post_id, 'reading_time', $read_time );
}
add_action( 'save_post', 'update_reading_time_meta_key' );

Could you help me with this?