Hide pages from the main menu

Instead of writing custom styles like .menu-item-346 where you are hardcoding some styles…

why not just add a custom css class with the editor under Appearance -> Menus.

WordPress has this build in.
Under Appearance -> Menus you need to open the Screen Options and tick the box CSS Classes.
After this you can now add one or multiple classes to every single menu-item.

You than could just write some custom CSS rules for these classes.
This way, your customer could just add these classes by himself. (or not)