Wordpress custom Walker menu - wordpress

Question

All day I'm struggling to create a custom menu walker which will fit the markup that must be on the frontend.
Here is the desired markup:
<nav class="menu cf">
<div class="menu__inner cf">
<ul class="menu__list">
<li class="menu__item menu__item--first">
Menu item 1
</li>
<li class="menu__item menu__item--has_children cf">
<a href="" class="menu__link">
Menu item 2
</a>
<a href="" class="menu__link--toggle">
<i class="icon-arrow-thin-down"></i>
</a>
<div class="sub_menu__wrap">
<div class="container">
<div class="gr-12 gr-3#lg">
<div class="sub_menu__heading">Menu heading (parent menu title)</div>
</div>
<div class="gr-12 gr-9#lg">
<ul class="sub_menu">
<li class="sub_menu__item gr-12 gr-4#lg">
Submenu item 1
</li>
<li class="sub_menu__item gr-12 gr-4#lg">
Submenu item 2
</li>
<li class="sub_menu__item gr-12 gr-4#lg">
Submenu item 3
</li>
<li class="sub_menu__item gr-12 gr-4#lg">
Submenu item 4
</li>
<li class="sub_menu__item gr-12 gr-4#lg">
Submenu item 5
</li>
</ul>
</div>
</div>
</div>
</li>
<li class="menu__item menu__item--last">
Menu item 3
</li>
</ul>
</div>
I have some examples of custom class that extends Walker_Nav_Menu
but I think those examples are no good.
Here is what I have so far: http://pastebin.com/qnPx9J67
IMPORTANT! I want to have BEM methodology on my classes, as like:
$menuListClass = 'menu__list';
$menuItemClass = 'menu__item';
$menuLinkClass = 'menu__link';
$subMenuClass = 'sub_menu';
$subMenuClass = 'sub_menu';
$submenuItemClass = 'sub_menu__item';
$subMenuLinkClass = 'sub_menu__link';
I also want to keep some default Wordpress classes, but in this format:
Examples:
.menu-item-has-children -> .menu__item--has_children
.current-menu__item -> .menu__item--current
I'm hoping someone could help me out with this since I don't know very good PHP. :)
Source link: https://developer.wordpress.org/reference/classes/walker_nav_menu/