[Add a submenu toggle button to WordPress menu items with children] Here are the PHP code and JS that will let you create a toggle and cause it to actually toggle classes and aria-expanded attributes. Use styles to show and hide the submenu, and to

filter.php

// Add this to an appropriate php file in your theme
// Change the function name and the "visually-hidden" class name as appropriate
// Note that this is a WireMedia theme, so is using their icons function.
// You can use css to create the chevron instead


require_once 'icons.php';

function j40a_parent_menu_item_buttons($output, $item, $depth, $args) {
  if (in_array('menu-item-has-children', $item->classes, true)) {
    $output = $output . '<button type="button" class="menu-item__toggle"
    aria-expanded="false" aria-controls="sub-menu-' . $item->ID . '">
    <span class="visually-hidden">Toggle Submenu</span>' . get_icon('angle-down') .
      '</button>';
  }
  return $output;
}
add_filter('walker_nav_menu_start_el', 'j40a_parent_menu_item_buttons', 10, 4);

menu-item-toggle.js

"use strict";

var menuDropdownToggles = document.querySelectorAll(
  ".menu-item-has-children > button.menu-item__toggle"
);

function toggleMenuDropdown() {
  if (this.getAttribute("aria-expanded") == "true") {
    this.setAttribute("aria-expanded", "false");
    this.parentNode.classList.remove("menu-item--open");
  } else {
    this.setAttribute("aria-expanded", "true");
    this.parentNode.classList.add("menu-item--open");
  }
}

// menuDropdownToggles.forEach(function (toggle) {
menuDropdownToggles.forEach(function (toggle) {
  toggle.addEventListener("click", toggleMenuDropdown);
});