The spec is the problem.

The delay is just irrelevant. It has nothing to do with it working ot not.

>b) no (cheap) way to iterate or deliver outside the spec

You could always do this in a waterfall project. Just make whatever changes to the code and ship. The problem is the same for SDD, as soon as you want quick changes you have to abandon the spec. Iterating the spec and the code quickly is impossible for any kind of significant complex project.

Either the spec contains sufficient details to make implementation feasible and iteration times become long and the process of any change becomes tedious and complex or the spec is insufficient in describing the complexity of the project, which makes it insufficient to guide an LLM adequately.

There is a fundamental contradiction here, which LLMs can not resolve. People like SDD, for a exactly the reason Managers like waterfall.