Divi | WordPress Theme | Mobile Menu Collapsible Submenus via Toggles | CSS & jQuery Tweaks in action: https://i.gyazo.com/93557e9ef5d4aad260e22c6d5896de3b.mp4

divi-mobile-submenu-toggles.css

/* when mobile menu is open, change hamburger icon to x icon */
#et_mobile_nav_menu .mobile_nav.opened .mobile_menu_bar::before {
	content: '\4d';
}

/* makes sub sub menu icon be right arrow instead of down arrow */
#top-menu .menu-item-has-children .menu-item-has-children > a:first-child::after, 
#et-secondary-nav .menu-item-has-children .menu-item-has-children > a:first-child::after {
	content: '5';	
}

/* - mobile menu toggling elements, injected via jQuery - */

/* make menu list item be relative, to be able to position toggle within this item */
#main-header #mobile_menu.et_mobile_menu .menu-item-has-children {
	position: relative;	
}
/* the new toggle element, which is added via jQuery */
#main-header #mobile_menu.et_mobile_menu .sub-menu-toggle {
	position: absolute;
	background-color: rgba(0,0,0,0.03);
	z-index: 1;
	width: 36px;
	height: 36px;
	line-height: 36px;
	border-radius: 50%;
	top: 4px;
	right: 4px;
	cursor: pointer;
	text-align: center;
}
/* the new toggle element when popped */
#main-header #mobile_menu.et_mobile_menu .sub-menu-toggle.popped {
	background-color: rgba(0,0,0,0.1);
}
/* toggle icon */
#main-header #mobile_menu.et_mobile_menu .sub-menu-toggle::before {
	font-family: "ETmodules" !important;
	font-weight: normal;
	font-style: normal;
	font-variant: normal;
	-webkit-font-smoothing: antialiased;
	-moz-osx-font-smoothing: grayscale;
	line-height: 36px;
	font-size: 24px;
	text-transform: none;
	speak: none;
	content: '\33';
}
/* toggle icon when triggered */
#main-header #mobile_menu.et_mobile_menu .sub-menu-toggle.popped::before {
	content: '\32';
}
/* hide sub menus by default */
#main-header #mobile_menu.et_mobile_menu .sub-menu-toggle ~ ul.sub-menu {
	display: none !important;
	padding-left: 0;
}
/* show sub menu when triggered via jQuery toggle, and add slight bg color */
#main-header #mobile_menu.et_mobile_menu .sub-menu-toggle.popped ~ ul.sub-menu {
	display: block !important;
	background-color: rgba(0,0,0,0.03);
}
/* remove sub menu list item left padding, since padding will be on anchors */
	#main-header #mobile_menu.et_mobile_menu li li {
	padding-left: 0;
}

/* adjust mobile menu anchors side paddings */
#main-header #mobile_menu.et_mobile_menu li a {
	padding-left: 20px;
	padding-right: 20px;
}
/* indent sub menu */
#main-header #mobile_menu.et_mobile_menu li li a {
	padding-left: 40px;
	padding-right: 20px;
}
/* indent sub sub menus further */
#main-header #mobile_menu.et_mobile_menu li li li a {
	padding-left: 60px;
	padding-right: 20px;
}
/* if mobile menu anchor has toggle, make room for it to fit next to the link */
#main-header #mobile_menu.et_mobile_menu .menu-item-has-children .sub-menu-toggle + a {
	padding-right: 44px;
}

/* - end mobile menu toggling elements - */

/* undo Divi's default styling of mobile menu links that have children */
#main-header #mobile_menu.et_mobile_menu .menu-item-has-children > a {
	background-color: transparent;
	font-weight: inherit;
}

/* make the current page's mobile menu link be different */
#main-header #mobile_menu.et_mobile_menu li.current-menu-item > a {
	font-weight: bolder;
}

divi-mobile-submenu-toggles.js

(function($) {
	function setup_collapsible_submenus() {
		$( "<div class='sub-menu-toggle'></div>" ).insertBefore( "#main-header #mobile_menu.et_mobile_menu .menu-item-has-children > a" );
		$( "#main-header #mobile_menu.et_mobile_menu .sub-menu-toggle" ).click(function () {
			$(this).toggleClass("popped");
		});
	}
	$(document).ready(function() {
		setup_collapsible_submenus();
	});
	$(window).load(function() {
		setup_collapsible_submenus();
	});
})(jQuery);

Add styles (without <style> tags) to your child theme CSS file
Add script (with <script> tags) to Divi Theme Options > Integrations > Body area

You may need to tweak the /* toggle icon */ font color and size of the toggle element, based on your colors and typeface.

This is great! I'm assuming there's no easy way to make the parent-items clickable as well? Ubermenu does ths, but I've noticed many menus seem to not allow top-level navigation items to be pages as well (making the toggle and text perform different actions).

@rezl , did you try this out? The parent menu items are clickable. That's one of the main reason I created this, instead of using other options I've stumbled upon.

great! thx, dude!

This is awesome. Worked perfectly. Huge improvement over default Divi mobile menu. Three years after you shared this, how has ET not made this standard?

I Dont know how to thank you man.
Perfectly working thanks again.
the code in divi documentation is useless
Awesome Job