Thank you, I needed to hear this stated clearly. The trend you point at closely tracks my own anxiety over time around having enough impact.
The relatively huge value of directing your whole career at EA is something that hasn't fully sunk in for me intuitively, and I expect the same for others who don't work at EA orgs.
Not financial advice. It's hard to give a specific recommendation since it depends on details of your personal situation and how risk averse you are. As a general rule, if your runway falling by 20% would still leave you with adequate runway (as defined by you), then investing it would likely be best for your long term wealth.
As you suggest, an index fund would be a very sensible, lower variance option.