Thanks everyone!
First, a few things I forgot to add. Choosing to go into engineering and then engineering management helped on the income side of things because those jobs tend to pay well. And other than the divorce I have been fortunate to not had any major financial setbacks -- my kids and I have been relatively healthy, I haven't been sued for anything, and my house hasn't been hit by any tornadoes or floods or fire.
@arebelspy and others who asked, I don't have any significant changes planned. My job is, in the big scheme of things, reasonably enjoyable, and I would not mind saving some more to increase my FI safety margin. Also, with my younger kids in middle school and junior high and parents nearby in declining health, I'll probably stick around here and keep doing what I'm doing for a few years.
While we were married, we did a lot of things right financially. We saved maybe 20% of our income in later years, and we managed to make a nominal profit on the three homes we owned (sequentially) due to good purchase decisions and a rising real estate market. So even though the divorce set me back by half, the half I was left with was still a significantly positive net worth. Over the last seven years, I basically ramped my savings up to about 50-60%, and my income over the past four years has been pretty good.
@Random, the doors that shut are the doors where I worry about reaching FI via inheritance - that won't happen because I did it without that. And the door of worrying about being 52 and getting laid off - ageism is alive and well in America, unfortunately. And the door of waking up and realizing I hadn't started saving for retirement or my kids college and facing another 15 or 20 years at a job while wanting to quit has shut as well. The door opening is just one where I can relax a little...now I can choose between having a financial disaster and still being better off than most, or quitting now and puttering around for the rest of my life, or working a few more years by choice to have a better standard of living. All three good choices.
@skyrefuge, my specific metric was when my annual spending rate (as calculated by my last three months spending in Quicken annualized) dropped below 4% of my current FIRE stash. I have been fortunate as well that my particular set of investments have been going up steadily for the past two weeks -- I'm mostly tied to the S&P 500 index. So I've managed to remain FI for about 10 days now, but yes, if the market swoons a lot or my expenses go up a lot, then I'll become un-FI again. If that happens, though, I'm still working and saving over half my income, so it should over the next few months or years become a permanent thing.
You also mentioned assumptions, which I think is an important point. In saying I have reached FI, I do make a number of assumptions. What I have tried to do in the area of assumptions is: (1) document them, (2) base them on reliable, long-term data when possible, (3) try to balance positive and negative ones, and (4) do sensitivity analyses to see which ones my financial plan really relies on. At the moment, I have nine negative assumptions where if my assumption is wrong it will make things worse for me; among them is "4% is a SWR at my age". I also have seven positive assumptions, where if my assumption is wrong my financial situation will be better than I expect; among them is "I won't obtain any more wealth from my stock options".