I think LLMs are actually great at breaking up work into workstreams and tasks. The issue is that tasks do not equal good PR stacks on their own. You’ll need to use feature slices instead (like do backend part -> PR it, etc). LLMs can do that, but you need to harness them into it.
I've had _some_ success by asking Codex write a full plan, broken up in to logical phases, complete with git commit points along the way.
It's probably not quite how I would approach doing git commits, but they're at least logical boundaries, and make narrative sense for a reviewer.