Yeah thanks Mathjak
I actually read a bunch of Kitces' articles right after I wrote that post, he has some really useful analyses with regard to SWRs and early trigger conditions!
Another good one of his that I found useful (prob a follow up to yours) is this one:https://www.kitces.com/blog/understanding-sequence-of-return-risk-safe-withdrawal-rates-bear-market-crashes-and-bad-decades/
Its worth reading through the whole article (don't stop after the first couple of graphs...there is more to the story).
But yeah, perhaps a careful look at that P/E Schiller ratio in the first few years of FIRE, and also, real returns in the first few years. If you look at this graph of his:https://www.kitces.com/wp-content/uploads/2014/10/Chart_Correlation-Graph.png
You can see that the best correlations with real returns (ignore nominal returns as he explains prior to that graph) is around the 8-10 year mark. However, you probably don't want to wait until the 8 year mark to make changes, if you happen to be in one of those bad runs! Otherwise you would have missed 7 years of correctional opportunity and it might be too late to really do anything about it (or at least, harder)!
Clearly, first year of FIRE, there is little correlation, so its worth just ignoring a bad real returns year then. Though the correlation gets much stronger in the second and subsequent years.
Kitces talks in many places (inc on that article) about how a diversified portfolio (he mainly keeps it simple with bonds/shares) helps to get a portfolio through these bad periods, particularly the first couple of years. I'm betting that, as well as having a well diversified portfolio (with international sharemarket diversification in different ETFs), if you just threw in a year of no withdrawals somewhere in the first few years, you would significantly reduce the chance of portfolio failure, at any WR above 4%. A "no withdrawal" year doesn't mean going back to work full time, it could just mean some part time work combined with some other moustachian way of reducing spending (eg maybe its working for an aid agency somewhere for a year on low pay). Essentially, having a no withdrawal year, would be akin to adding another factor of diversification to your portfolio.
It would be interesting to see the effect on SWRs, if a "non withdrawal year" is added somewhere in the first ten-fifteen years of draw down. Or 1.5 years, or two years. Given the many, many moustachian ways of doing such a period, its not outside the realms of FIRE possibility for most people on MMM.
The main way I can think of analysing this is by going through a TON of simulations on cfiresim or Firecalc. But there is probably a smarter way to model it by using a script. If going the cfiresim/firecalc route, you would have to do runs for different lengths of fire (I would suggest 30, 40, 50 years...being thus realistic for early FIRErs), various withdrawal rates (eg 4, 4.5, 5, 5.5,.....9%), and then insert a year of zero spending (probably by adding a years income) in each year from 1 up to R where R = years of retirement. It could then all be repeated again with a second and third year of zero spending (but using all possible combos, eg, your zero spending could be consecutive ie FIRE years 3,4,5, or non, eg years 1,4,6)...and it could be repeated with some different portfolio allocations (to the limits of what those two tools provide...).
(As you can see that's a lot of simulations, really, would be easier with a script).