Going much deeper on tests has been transformative for me. In a solo project started from scratch, I'm 6-7 weeks in, and it's up to ~90K lines. ~60K of those are tests. Those tests have now found (and then the agent has correctly diagnosed) multiple bugs in broadly used libraries that I'm using in my project. That's because those bugs surfaced as occasional issues in my project. Especially powerful are all the property-based tests (perhaps what you are calling fuzzing? I'm using the Python package called Hypothesis for this).

Another spectrum that I've found useful to explore is the scope of what I ask the coding agent to do in one turn. I see some people trying to do one massive prompt that the coding agent works on for a day or more. I find a large boost in overall quality if I do 10-20 prompts per day (not counting the prompts where I'm just trying to understand things). It's still much less of my time than hand-coding, but the resulting architecture looks like my own. The quality of the overall system is great. There are certainly issues here and there in the code, but it's always that way once a project gets large enough. Now it's easier to address any particular issue throughout the code base in one go.