Many people around me are saying US based index funds is enough coverage to FIRE but I want to know if it's worth diversifying even more, maybe 10-20%?
There is a research paper by Vanguard titled "Global equity investing:
The benefits of diversification
and sizing your allocation"
On page 5, it shows that for a U.S. investor, the best amount of international exposure for ideal risk adjusted returns is around 25% of your portfolio. Based on their research, I think its wise to have at least a little outside of something like an SP500 fund.