Get post meta in function hooked to transition_post_status

Question

I have a simple function hooked to transition_post_status: it must only fire when the post status is "publish":

function on_publish($new_status, $old_status, $post)
{

    // (...) omitted code

    if (('draft' === $old_status || 'auto-draft' === $old_status) && $new_status === 'publish') {

    // problem here
        $checkbox_value = get_post_meta($post->ID, '_custom_checkbox', true);
        $textarea_value = get_post_meta($post->ID, '_custom_textarea', true);
    }

}

add_action('transition_post_status', 'on_publish', 10, 3);

Then I have a simple metabox with a checkbox and textarea. I need the data in my transition_post_status, but I can't retrieve them. This is the code for the metabox.

add_action('add_meta_boxes', 'custom_metabox_add');
function custom_metabox_add() {
    add_meta_box(
        'custom_metabox',               
        'Impostazioni Personalizzate',  
        'custom_metabox_callback',      
        'post',                         
        'side',                         
        'default'                       
    );
}

function custom_metabox_callback($post) {

    $checkbox_value = get_post_meta($post->ID, '_custom_checkbox', true);
    $textarea_value = get_post_meta($post->ID, '_custom_textarea', true);

    echo '<p>';
    echo '<label for="custom_checkbox">';
    echo '<input type="checkbox" id="custom_checkbox" name="custom_checkbox" value="1"' . checked(1, $checkbox_value, false) . ' />';
    echo ' Enable';
    echo '</label>';
    echo '</p>';

    echo '<p>';
    echo '<label for="custom_textarea">Notes:</label>';
    echo '<textarea id="custom_textarea" name="custom_textarea" rows="5" style="width:100%;">' . esc_textarea($textarea_value) . '</textarea>';
    echo '</p>';

    wp_nonce_field('custom_metabox_nonce_action', 'custom_metabox_nonce');
}

add_action('save_post', 'custom_metabox_save');

function custom_metabox_save($post_id) {

    if (!isset($_POST['custom_metabox_nonce']) || !wp_verify_nonce($_POST['custom_metabox_nonce'], 'custom_metabox_nonce_action')) {
        return $post_id;
    }

    if (defined('DOING_AUTOSAVE') && DOING_AUTOSAVE) {
        return $post_id;
    }

    if (!current_user_can('edit_post', $post_id)) {
        return $post_id;
    }

    $checkbox_value = isset($_POST['custom_checkbox']) ? 1 : 0;
    update_post_meta($post_id, '_custom_checkbox', $checkbox_value);

    if (isset($_POST['custom_textarea'])) {
        update_post_meta($post_id, '_custom_textarea', sanitize_text_field($_POST['custom_textarea']));
    }
}

What can I do to have the values saved before I call on_publish?

Any help is appreciated.

[update]

I tried doing the following, that is having another function hooked to transition_post_status but with higher priority. It works, but I don't like it too much.

function dirty_transition(){
    global $post;
        if(!empty($post->post_type) && $post->post_type == 'post') {
            update_post_meta($post->ID, '_custom_checkbox', isset($_POST['custom_checkbox']) ? 1 : 0);
            update_post_meta($post->ID, '_custom_textarea', isset($_POST['custom_textarea']) ? sanitize_text_field($_POST['custom_textarea']) : '');
        }
    }

  add_action('transition_post_status', 'dirty_transition',1,1);