Rather than donating 5-6 figures, what about saving enough cash to live on? Roughly speaking you can "make" ~$200k/year on $5M in T-bills. You could live comfortably in the US or Europe, or basically like a king anywhere else in the world. You could work on the software you want, even pro-bono, and walk away any time. I believe this is called "F U money".

My initial offer was $400k total comp (E5), so I didn’t really consider FIRE as an option in the near term considering my spouse is a full-time parent and this area is HCOL.

I’m nowhere close to $5m, and I’m hoping to leave Meta in the next few months. But I’d love to be able to “retire” and work exclusively for companies that match my morals.

I figured the amount I’m donating doesn’t make a huge difference to my FIRE date

That 200k is a reasonable amount to start withdrawing from a 5M portfolio (exactly the 4% rule from the Trinity study [1]), but you’ll want to adjust it for inflation every year. My favorite tool for planning these strategies is TPAW Planner [2], which visualizes the distribution of withdrawals under various market outcomes. It’ll also suggest a portfolio of stocks and bonds that’ll be safer than just T-bills, which have a high risk of not beating inflation.

1: https://www.bogleheads.org/wiki/Safe_withdrawal_rates

2: https://tpawplanner.com/