Drupal 8 One-off Custom Page for Theme
You might want to have a look at the Paragraphs project. It basically allows you to create different building blocks (Image with text, Diagram with description, ...) and then you can piece them together in the edit form pretty easily.

Check out the demo module that's part of the project for a quick start. You'll still have to actually implement the design for each building block but once you have that, you can easily change how you've combined them and move parts around, add new ones.. create multiple pages that each uses those building blocks differently.