Replace the CSS with this:
@media (max-width: 768px) {
.secondary-navigation {
text-align: center !important;
overflow-x: scroll;
-ms-overflow-style: none;
scrollbar-width: none;
scroll-snap-type: x mandatory;
}
.secondary-navigation ul {
display: flex;
}
.secondary-navigation .sf-menu>li {
flex: 1 0 120px;
scroll-snap-align: center;
}
}
.secondary-navigation .menu-toggle {
display: none;
}
You can adjust the Menu Item Width in Customizer > Layout > Secondary Nav.
And in the above CSS this line: flex: 1 0 120px;
sets the min width of each item.