Hide menu item when users are logged in/out

I should suggest a custom walker to, not that hard at all.

starting with:
https://developer.wordpress.org/reference/functions/wp_get_nav_menu_items/

in the foreach loop you are able to check based on title.

in this part you will use:
https://developer.wordpress.org/reference/functions/is_user_logged_in/

foreach($nav_items as $item): 
if($item->title == 'Register'):
if(is_user_logged_in()):
//Do your thing when logged in
else:
//Do your thing when not logged in
endif;
endif;

endforeach;

Above is an example, there are multiple ways you can do this 🙂 hope it helps you