I'd probably recommend https://old.reddit.com/r/FatFIRE over /r/pf as it's more likely to have people that have experienced getting a large lump sum like this.
You can use a variety of online calculators to back test a 4% withdrawal rate - maybe 80% safe, but 20% of the time you'll go broke before dying.
The first is going back to the origins of the 4% number in the first place, the trinity study. The parameters for that were a 30 year retirement period, and success was "not completely run out of money after 30 years, 95% of the time". If you extrapolate from that original study, if you retire for more than 30 years (FIRE includes retiring early), success drops from 95%.
There's articles exploring that, e.g.
https://www.fiphysician.com/safe-withdrawal-rate-early-retir...
https://www.madfientist.com/safe-withdrawal-rate/#:~:text=Th...
And then there are a variety of online calculators where you can play with the numbers yourself.
The other elephant in the room is pre-Medicare healthcare costs.