Maybe this is a different use case for code generators from what you have in mind? This isn't one-time upfront scaffolding, it's a recurring part of the development process.
When email templates are edited in the external service, you run the generator to pull in the latest structure, and it breaks your build with new type errors.
When email templates are edited in the external service, you run the generator to pull in the latest structure, and it breaks your build with new type errors.