There's a thread elsewhere in the forum that illustrates the value of diversification. It's called "Idiots vs Gurus". From 1970 to today, in multiple countries, it appears that "randomly" picking a wide variety of investments that include many stocks but also several other investment types has delivered a better return than most of the portfolios carefully selected by experts. The diverse portfolios were especially good at avoiding really bad downturns. They suffered in bad times, but not as much as the narrowly selected portfolios of the experts.
https://forum.mrmoneymustache.com/investor-alley/portfolio-design-idiots-v-gurus/Investing this way would take a bit of learning at first (it's initially a confusing read). After that, probably a bit more tax work than a single-asset choice. But you could do the whole thing in an afternoon a year pretty easily if you wanted, and you might get a better risk/ reward ratio than just a stocks n bonds mix.
If I recall correctly, the menu of "random" choices had a proportion of roughly:
35% US stock
37% foreign stock
15% bonds
13% real assets (gold, REITs...)
Cash was option too. I don't think it's crazy to have some of the bonds portion in cash with interest rates so low. In any case, it seems that diversifying within the stock portion was helpful and diversifying within the non-stock portion was also helpful. By doing both, in the past the high-ish stock % was safe enough to do well despite occasional dips.
No one can be sure of the future. Diversifying gradually myself. Already FIREd with stash of surprisingly similar size, though owning a house. Good luck!