I too seem to have ended up with oodles of layers of security, all of which go to reinforce each other.
Firstly, we tried living on our 'what would we have if we retired now' budget well before we actually FIREd and then worked a couple of unnecessary extra years to be extra safe. I probably should have gone earlier, but the extra helped me sleep.
Secondly we have very low fixed expenses thanks to a paid off house. We have plenty of cheap hobbies and can be very happy living on much less than our planned budget (which included lots for travel.) The pandemic reinforced this.
I always planned a fixed percentage withdrawal strategy to eliminate SORR. We were happy we could survive if markets / our drawings dropped by 50% and had cash buffers to ensure that our actual spending would only have to drop by half of that for quite a few years. One year of cash can turn 4 years of 50% slump into just a 25% drop in spending.
I agreed to keep working one day a week post FiRE, 100% wfh, for a higher rate and with paid overtime if I needed to do more than my 7 hours. Surprisingly I'm still doing and enjoying this after 4 years of FIRE. These earnings covered 100% of spending for the first 3 years thanks to overtime in year one and low spending due to covid in years two and three. I plan to keep doing this as long as I enjoy it / they want me.
We have pension due between 11 and 15 years after our 2018 FIRE date that would cover our core expenses. After not spending any of our bridging funds, we now have cash to cover the remaining gap to the pensions. That means we'd actually survive now if our investments went to zero!
Crazy safe.