Capital gains are part of your income, so realizing the gains themselves will take you over the limit unless total profit including gains and dividends is < $40k-ish.
You might be able to take out small amounts strategically to avoid the 40k threshold.
I may be mistaken, but isn't FIRE telling people to live off 4% of their investments forever? If the goal is 1 million, that is exactly 40k.
The math works out perfectly to leech on society - and
(this is just a hypothesis) possibly causing S&P 500 ETFs to inflate as these FIRE "retirees" promote their methods online, creating a feedback loop and illusion of ever-increasing stock market boom for the last 10 years
