Thinking outside the box, what other methods do you use to determine your FIRE readiness?
I think that question is more important than finding other modeling tools. This thread, started yesterday, talks about figuring out how you will generate income from your assets in retirement, or another way of knowing when you're ready.
https://forum.mrmoneymustache.com/welcome-to-the-forum/forget-the-4-swr-or-any-swr-it's-all-about-income/I maintain that as long as you are relying on a 100% "paper" portfolio, you're going to have to accept that you will be heavily relying on historical data to make an educated guess at future success. And, you'll likely be experiencing some pretty wild ups and downs along the way if you are invested heavily in stocks (which is pretty much a necessity for successful outcomes in cfiresim scenarios). Models like cfiresim are super helpful, but they can only get you so far. I'm a fan of finding a safe method to generate guaranteed income to cover at least your minimum expenses for the rest of your life, so you're not worrying about going broke or decreasing your standard of living -- or God forbid finding yourself old and your plan crapped out. Some of those usual methods are purchasing an annuity (assuming you don't already have a pension from work), Social Security income, and rental income (which generally is a steady, appreciating cash flow source and a great hedge against inflation).
If you can find some ways to make your net worth generate income, using reasonably conservative assumptions, then I think folks will find they have a much higher confidence in their long term success. You won't be relying much on models, but instead will be looking at almost exactly how much income you will have coming in with a very high confidence. But if the plan is to rely on paper assets, then we have to accept the chance (even if small) that there could be some future periods that look nothing like history, and then the models will have proven to be very limited in their ability to predict outcomes. For example, how well would a 100% equities portfolio have survived Japan's recent 20-year stagflation period?