Interesting... I've spent 10 years crafting HTML emails. Your HTML templates look good in dark mode as well. Is there any chance to preview the dark mode without sending a test email?
I believe that JSON Schema is a great solution because it is a standardized format and provides helpful error messages for validation issues. If the schema file is well-defined, it should be safe to map the JSON data to a static type DTO and trust that the data types are valid. JSON Schema's ability to validate strings, numbers, integers, and custom objects makes it a powerful tool, and I personally wouldn't want to attempt to implement something similar on my own.
Why should people use just one tool all the time? There are many alternatives in this world, free as well.