Templates as content
It sounds like what the client really wants is something like the Paragraphs module:

https://www.drupal.org/project/paragraphs

It's a good solution for creating flexible, yet structured, content.

Take a look at this video for a good idea of the promises (and pitfalls) of this approach:

https://www.oomphinc.com/notes/2018/11/component-based-design-system-drupal-8-paragraphs/