> It’s the picture book waterfall implementation.

One of my better experiences with software development was actually with something waterfall-adjacent. The people I was developing software for produced a 50 page spec ahead of any code being written.

That let me get a complete picture of the business domain. That let me point out parts of the spec that were just wrong in regards to the domain model and also things that could be simplified. Implementation became way more straightforwards and I still opted for a more iterative approach than just one deliverable at the end. About 75% of the spec got build and 25% was found to be not necessary, it was a massive success - on time and with fewer bugs than your typical 2 week "we don't know the big picture" slop that's easy to get into with indecisive clients.

Obviously it wasn't "proper" waterfall and it also didn't try to do a bunch of "agile" Scrum ceremonies but borrowed whatever I found useful. Getting a complete spec of the business needs and domain and desired functionality (especially one without prescriptive bullshit like pixel perfect wireframes and API docs written by people that won't write the API) was really good.

If you can't get a complete spec, it's better start with something small that you can get detailed info on, and then iterate upon that. It will involve refactoring, but that is better than badly designing the whole thing from the get go.