That's a great approach, though I'd also recommend setting up a strong basis for linting, type checking, compilation, etc depending on the language. An LLM given a full test suite and guard rails of basic code style rules will likely do a pretty good job.

I would find it a bit tricky to write a full test suite for a product without any code though. You'd need to understand the architecture a bit and likely end up assuming, or mocking, what helpers, classes, config, etc will be built.