I didn't have to. I funded my kids 529s before I FIREd.
If I were in your shoes, I'd divide the question into three parts:
1. How much to fund their college? I picked a target number when my kids were young and funded to that level, then stopped. The last five to ten years I've just been monitoring my 529 balances and trying to hit zero when my last kid graduates.
2. Whether to fund that via 529 or taxable (or Roth, which some people like - I don't). I chose to fund as much as possible via 529s, ESAs, and UTMAs. I thought the tax advantages were worth it, but my state also gives a state tax deduction for up to $6000 per year of contributions, which certainly factored into my planning. I also discovered that the 10% penalty, especially with multiple kids who are all likely to go to college, is quite manageable / avoidable via a number of techniques.
3. Where to get money from to pay for things? Dividends, interest, capital gains, Roth distributions, whatever. Now that my kids are in college, I generally pay for their college via (1) scholarships, (2) financial aid, (3) AOTC / out-of-pocket, (4) 529s.
These three questions are largely independent, so it might be easier for you to analyze them separately.