I've not seen the 403/404 error page customisation in Joomla 5 - where do you configure these?
404 and offline COULD be a fully edited layout but a custom article and menu item is probably easier to use as you could load a basic template and assign it to the menu item and you'd be away!
Thinking about it a custom menu item (which could be a single article view) would be the easiest to implement for both 404 and offline.
What do you think in a 404 situation - should we redirect to a new menu item or display the custom error page 'in situ' ?