You could do something like this:
Create a new custom menu item.
Give it a custom class eg. custom-toggle
Add this is as your menu item label:
<span class="gp-icon icon-menu-bars"><svg viewBox="0 0 512 512" aria-hidden="true" role="img" version="1.1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="1em" height="1em">
<path d="M0 96c0-13.255 10.745-24 24-24h464c13.255 0 24 10.745 24 24s-10.745 24-24 24H24c-13.255 0-24-10.745-24-24zm0 160c0-13.255 10.745-24 24-24h464c13.255 0 24 10.745 24 24s-10.745 24-24 24H24c-13.255 0-24-10.745-24-24zm0 160c0-13.255 10.745-24 24-24h464c13.255 0 24 10.745 24 24s-10.745 24-24 24H24c-13.255 0-24-10.745-24-24z"></path>
</svg>
</span>
Then use this CSS to hide the dropdown toggle icon for the sub menu:
.custom-toggle .dropdown-menu-toggle {
display: none !important;
}