For me, the main tradeoff is not compensation, but time. I'm prepared to trade away compensation for fewer hours.
But if I want to work a maximum of 32 hours instead of 40, then I essentially have to rule out most of the jobs I would otherwise apply for. It's not a criteria where most employers offer flexibility.
When I left there (for reasons), this was something I discussed with all three places that made me an offer. IIRC, one offered me 32 hours a week, another 32 (I think), and the third 36. I ended up accepting the 36 because it seemed like the biggest challenge for me.
I brought up my desire to work < 40 hours a week as early as possible with places I interviewed, and some just said "no way", which ended the interview process quickly.
For context, I'd been in software dev about 18 years when I was going through my last set of interviews.
Anyway, I'm just responding to say that it's possible to find places willing to do this, though obviously the more desirable you are as a candidate the more likely you are to find an employer willing to go for this.
The other alternative is to try to build up a consulting practice, and to simply not take on that much work. But this is a lot more stressful from a financial security perspective, having done this for a bit myself many years ago.
I pitched my boss that I would get all my work done AND it would be good for him because it would help him meet budget AND build resiliency in the team (They have to deal with outages without me - so I created more documentation around that). After lots and lots of back and forth my boss eventually accepted, and I never worked more than 4 days a week again, and most weeks it was only 3 days.
Keep pushing for it, and find a company/boss who is open to the idea. Fire your current one if they're not willing to negotiate.
The main issue is a lack of institutional support:
• While there are a decent number of potential employers who would be open to 32-hour-full-time, there's not a good way to reach them. RIP 30hourjobs.com I'm afraid.
• Recruiters are completely baffled by the idea that "full-time" doesn't necessarily mean 40 hours, even if the IRS determination is minimum 30/week or 130/month: https://www.irs.gov/affordable-care-act/employers/identifyin...
• Many companies simply don't have the HR flexibility to handle anything other than 40-hour full-time cogs. (I think this may be different in parts of Europe, where I understand there are laws that allow employees to choose to work fewer hours, so companies have to find a way to accommodate them.)
But I also have family obligations that I cannot set aside, and I can't do all of these things without burning myself out. Fortunately I am in a position where I can afford to sacrifice some compensation — and that's what I choose to do.
So even setting aside your point about the marginal value of the last 8 hours (which rings true), in my case there is definitely no question about the value of each hour.