I am familiar with creating node and pointing to that specific node in basic site settings. How do i create this programmatically and package in features? Twig override?
To bundle and create programatically, you would need to create a custom module and inside that module: you could create a route for each specific page. That module could also update the config that is found in the system basic settings to use your custom pages.
Using this custom page you could call a custom page.html.twig and override with the html you need.
Here are two resources:
With regard to the 400 class errors it looks like this is possible (but not yet easy) with Drupal 8 - see https://www.drupal.org/node/2363987
It will need someone to write a contrib module to make this a simpler process at this stage.
A 503 error may not allow Drupal to return a complete page, or any page at all, so one might normally rely on a web server, cache server or CDN to serve a page result up for this kind of response.