Earlier quoted context omitted.
Not really, no company hires you for 10 or 20% in software engineering, while still paying enough that you could sustain yourself. The minimum is 80%, because it still the most of your time, so it seems reasonable in the work oriented society.
You can definitely get contracting gigs (Upwork at the bottom tier) or consulting work to work only 10-20% hours. Depending on your skill set you might find it easier to do “blocks” of 20-hour weeks to align with client needs, then take blocks of time off. It’s doable, but not on a standard employment contract.
No idea how to deal with the uncertainty of being able to get enough contracts that pay enough.