OK you have this CSS which makes up for the missing dropdown toggle:
.main-navigation .main-nav ul li.menu-item-has-children > a {
padding-right: 10px;
}
Make it: padding-right: 20px;
to march the menu spacing.
For an underline – use this CSS instead of your current:
@media (min-width: 769px) {
.main-navigation .menu>.menu-item>a::after {
content: "";
position: absolute;
left: 20px;
right: 20px;
bottom: 15px;
height: 2px;
opacity: 0;
background-color: currentColor;
transition: 0.3s opacity ease;
}
.main-navigation .menu>.menu-item.current-menu-item>a::after,
.main-navigation .menu>.menu-item.current-menu-ancestor>a::after,
.main-navigation .menu>.menu-item>a:hover::after {
opacity: 1;
}
}