I can't afford to leave my job, I would only be able to sustain myself for ~1 year. I think I would be satisfied if I only had to work half as much: 4 hours a day, 20 a week. Now I just need to find that job...
So basically it is possible, and I think in the US too you can negotiate to work less hours.
Really bluntly, just as value-per-hour starts falling off rapidly after some point, it also increases super-linearly up to some point before that. Most overhead is constant regardless of if you work 24 or 40, you'll miss opportunities when you're not there, and you'll need to spend more time catching up when you are. That doesn't mean you can't work effectively, but I've watched multiple junior-mid devs think they've hacked the system but instead just forgo most learning/advancement opportunities.
If you're happy with the current salary and don't need any more fulfillment from your career than turning-the-crank, I guess go for it.
(In the US they'll just fire you, maybe even just for asking.)