If your startup needs no funding, what is the use of a detailed business plan?
You may not need a “business plan”.
But you sure need an operating plan — a very detailed financial model.
If you don’t build an operating plan, and track against it, what I can almost guarantee you is costs will run away from. Not necessarily in big chunks, but an extra expense here, a contractor there.
Combine bigger expenses with a miss on revenue, even a minor one — and you can quickly get into trouble.
Build a real operating model / plan, at least in a Google sheet. And have your finance person (even if she’s just a 4-hour-a-month person) track vs. the plan every month.
It will keep you much more on track.