What framework guides a late-30s investor's asset allocation beyond stocks?

[Luke.W]

I'm in my late thirties and finally have a stable income, allowing me to move beyond just contributing to my 401k and think more strategically about my overall asset allocation. My portfolio is currently very stock-heavy, mostly in low-cost index funds, and I know I should diversify, but I'm uncertain about the right mix of bonds, international exposure, and maybe a small alternative investment slice for my age and risk tolerance. For those who have crafted a long-term allocation plan, what framework or principles did you use to decide on your percentages? How do you balance the desire for growth with the need for stability, and how often do you rebalance or reassess your strategy in response to major life changes or market shifts?