Have you read the book "Quit Like a Millionaire" by Kristy Shen and Bryce Leong yet?  They offer a pretty compelling approach to achieve FIRE.

Their broad FIRE strategy is as follows:

Have a global medical insurance policy in place. Determine investment portfolio needed - i.e. Annual_Expenses x 25. Invest in low-expense Exchange Traded Funds (ETF) that track indices for a balanced equity-bond portfolio.   Weigh it more heavily with higher-yielding ETFs for the initial 5 years to reduce the negative impact of retiring in a downturn market - i.e. mitigate against sequence-of-return risk. Perpetually withdraw at a 4% rate per year during retirement. Income yielding portfolio provides a Yield Shield - i.e. even if the market tanks, there is still distribution. Create a Cash Cushion to cover the shortfall in those bad years. Bad years typically did not last beyond two years before the market recovers to previous levels. 5 years is...