> I've worked under 40 hours a week and rarely worked overtime
That is exactly the slack in the system that makes the endeavor sustainable. There's no way to get quality software out of consistently drained engineers.
Trying to find some "balance" without hedging for the asymmetry inherent in the problem will lead you to burn out roughly half of the workforce, which has nonlinear (very superlinear) knock-on effects for the success of the remaining half.
If you burn out the top half of your workforce, the bottom half will suddenly bear twice the load and burn out that much faster.
That said, I've been a software engineer for over twenty years and most of those years I've worked under 40 hours a week and rarely worked overtime.