How to set parent custom classes in wp_nav_menu - WordPress

Question

I am trying to emulate the following navigation using wp_nav_menu ( https://developer.wordpress.org/reference/functions/wp_nav_menu/ ) in WordPress.

My demo menu structure:

Menu
>child1
>child2
>>submenu1
>>>subchild1
>>>subchild2

<nav class="collapse">
<ul class="nav nav-pills" id="mainNav">
 <li class="dropdown">
 <a class="dropdown-item dropdown-toggle" href="#">Page1</a>
    <ul class="dropdown-menu">
        <li><a class="dropdown-item" href="#">child1</a></li>
        <li><a class="dropdown-item" href="#">child2</a></li>
        <li class="dropdown-submenu"><a class="dropdown-item" href="#">submenu1</a>
                <ul class="dropdown-menu">
                    <li><a class="dropdown-item" href="#">subchild1</a></li>
                    <li><a class="dropdown-item" href="#">subchild2</a></li>
                </ul>
        </li>
    </ul>
 </li>
</ul>
</nav>

I am using the below code and was able to apply all the classes to the menu except for the dropdown-toggle and dropdown-submenu classes. I need to apply the dropdown-toggle class to the first <a class if it's a parent and I need to apply dropdown-submenu to the li's that are submenus with children

<?php
wp_nav_menu(
    array(
        'theme_location'  => 'primary',
        'container' => 'nav',
        'container_class' => 'collapse',
        'container_id'    => '',
        'menu_class'      => 'nav nav-pills',
        'fallback_cb'     => '',
        'menu_id'         => 'mainNav',
        'link_class'      => 'dropdown-item',
        'depth'           => 4,
        'walker'          => new Understrap_WP_Bootstrap_Navwalker(),
    )
);
?>

I tried using:

    function add_menu_parent_class( $items ) {
    $parents = array();
    foreach ( $items as $item ) {
        //Check if the item is a parent item
        if ( $item->menu_item_parent && $item->menu_item_parent > 0 ) {
            $parents[] = $item->menu_item_parent;
        }
    }
    
    foreach ( $items as $item ) {
        if ( in_array( $item->ID, $parents ) ) {
            //Add "menu-parent-item" class to parents
            $item->classes[] = 'dropdown-submenu';
        }
    }
    
    return $items;
    }
    
    //add_menu_parent_class to menu
    add_filter( 'wp_nav_menu_objects', 'add_menu_parent_class' );  

...in my functions.php but it applies the dropdown-submenu class to all the parent li's as opposed to just the submenu ones.

I was hoping someone may have a a PHP or jQuery solutions for this. My goal is to reverse engineer the wp_nav_menu into a theme I have as opposed to styling the classes wp_nav_menu generates by default. I'm just a WP enthusiast so trying to learn as I go. Thanks