I don't think they are mutually exclusive, that if you take one you give another. TBH, its more about the team you work with than startup vs big-tech company thing.

I don't know how many of big-tech have flat hierarchy, at-least not the ones I've worked at and the people/team culture have definitely better. Were there times there was bureaucracy and all, sure, but I personally would take take good WLB most of the year, over fucking up my health/relationships over making some random software on claude code all night that these days anyway most want to be AI slop. If I've the energy to work extra, I'd work on my own hobby software projects.

That said, this or that, its all personal preference. So no wrong way of life here. Whatever, you want your life to be.