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.