I find that my bottleneck with LLMs on a real project is reviewing their code, QAing it, and, if it's novel code, integrating it into my own mental model of how the code works so that I can deliberately extend it in a maintainable way.

The latter is so expensive that I still write most code myself, or I'll integrate LLM code into the codebase myself.

I've used parallel Claude Code agents to do chore work for me.

But I'd be curious about examples of tasks that people find best for OP's level of parallelization.