The way many people I've known seem to achieve flexibility is to accept a role somewhere relatively laid back, typically at a large but non-tech corporation. If they're coming from a much more intense environment and working remotely, it often requires only a couple of hours work a day to over perform, if that.

It seems like it's almost impossible to get a role that's explicitly part time (as you've experienced) but plenty of companies are willing to offer lower salaries while turning a blind eye to enforcing the supposedly 40 hour work week.

The other ways seem to involve some combination of working for yourself, consulting, and short periods of full time employment interspersed with not formally working.