How can I inject relevant/filtered ads every 5th result in a view?
What you could do is

Create view only for ads:
Create preprocess function in your custom theme/module that is loaded before the template.
In this preprocess function call your view programmatically with the filters from the current page.
In Drupal 8 you can set filter programmatically like that:


ViewExecutable::setHandler($display_id, $type, $id, $item);


Order the results in the way you want and store them in an array variable that you can access from the twig.

Hopefully that helps.