WordPress add sub-menu toggle buttons to any menu.

filters.php

<?php

function render_sub_menu_toggle( $item_output, $item, $depth, $args ) {

  $children = false;

  // If menu is using a walker.
  // Walker class has children method.
  if ( $args->walker->has_children ) {
    $children = true;
  } else {
    // Else have to test for children.
    $items   = wp_get_nav_menu_items( $args->menu->term_id );
    $parents = array_map(
			function( $item ) {
					return is_object( $item ) ? $item->post_parent : $item['post_parent'];
			}, $items
		);

    if ( in_array( (int) $item->object_id, $parents ) ) {
      $children = true;
    }
  }

  if ( $children ) {
    // Load sub menu script.
    // First wp_register_script in functions.php.
    wp_enqueue_script( 'sub-menu-js' );

    $title = $item->post_title ?: $item->title;

    $item_output .= "<button class='sub-menu-toggle' value='Show sub menu for $title'></button>";
  }

  return $item_output;
}
add_filter( 'walker_nav_menu_start_el', 'render_sub_menu_toggle', 10, 4 );

sub-menu.js

;(function() {
	'use strict'

	const toggles = document.querySelectorAll('.sub-menu-toggle')
  if ( ! toggles ) {
    return;
  }

	for (let index = 0; index < toggles.length; index++) {
		let menuToggle = toggles[index]
		menuToggle.addEventListener('click', toggleSubMenu)
	}

	function toggleSubMenu(e) {
		let menu = this.parentNode
		menu.classList.toggle('sub-menu-open')
	}
})()