> Weird to be working on a 10k codebase that didn't exist a few hours ago

This is the issue. A firehose of code production is not useful for a long time. But it's very useful in short bursts. Total volume of code was never the bottle neck or the goal.

Working with LLMs has all the same pitfalls of working with people. Programming effectively for the long term requires all the same rules whether you're using LLM's or not: build incrementally, test and refactor as you go (not at the "end"), keep scopes small, ship frequently, etc.