Secondary Off Canvas Panel

I added it, but instead of replacing the "Menu" name it adds an additional line on top as the category name.

If I remove the Mobile Menu Label in customiser, then the entire menu disappears.

Ok remove the hook and add this PHP Snippet to your site:

add_filter( 'option_generate_secondary_nav_settings','lh_single_posts_secondary_nav_settings' );
function lh_single_posts_secondary_nav_settings( $options ) {
    $category = get_the_category(); 
    $category_label = $category[0]->cat_name;
    if ( !empty( $category ) ) {
      $options['secondary_nav_mobile_label'] = $category_label;
    }
    
    return $options;
}

Works perfectly. Thank you!

I will mark this topic as resolved, since the original question has been answered.

Glad to be of help.
And thanks for sharing your solution.