Yes I have! Though slightly more FTE than that, and due to mix of caring responsibilities and long-term health issues with similar reasons to those you have given.
Moving to PT working definitely does work for me as way of reclaiming my life from overwork - mainly because it acts as a very effective tool to say no to extra stuff (which, as you will know from bitter experience, just tends to endlessly accumulate when you're FT in academia). Whatever the extra thing under discussion is - say a meeting taking place on a day you are not contracted to work, or a responsibility that would take you over your allotted hours - you can simply say no. If you have been used to being walked over for years, that act of saying no without guilt is very empowering. So too is putting up your autoreply on your days off.
It's important, though, to clearly agree with your line manager beforehand which existing responsibilities exactly you are giving up (e.g. modules x + y + admin responsibility z). But if you occasionally want to spend a day off doing some research, knowing that you are doing it as a hobby for personal interest, not for your employer, also feels empowering. Finally, for tax reasons, your monthly take home pay will be reduced by slightly less than your hours are, and you'll also save on commuting/lunch/union subs.
Frankly I wish I'd made the move years ago when my family were younger (though I couldn't really have afforded to then). I'm a dad by the way - I don't know any other male academics who've taken this step, but more dads working PT has got to be good for gender equality.
An additional advantage that I hadn't considered before going PT is that, if in future you do want to earn some extra cash, you could temporarily take on some hourly paid teaching work for another university. Again very empowering (if not very lucrative), as you would be doing the extra work on your terms, and somewhere you aren't taken for granted, rather than feeling a sense of having to give everything to your main employer to justify your FT salary.
One caveat however: I would be a bit more hesitant to recommend PT working to a HoD, as I imagine that the nature of the job means that decisions sometimes need to be made at unpredictable times, so there is a danger you could indeed end up working more hours than you are paid for. So if I were you, I would be looking for a way to resign from HoDship as part of your PT move.
But frankly given that most academics work more hours than they are paid for anyway, even if you can't immediately give up the HoD position it would be much better for your wellbeing to say work 0.7 FTE hours for 0.5 pay rather than work 1.2 FTE hours for 1.0 pay and feel exhausted the whole time.
I totally agree that it's healthiest to treat academia as a job. To some extent it's also a vocation for me, but I don't identify with the dominant idea today of academia as a career. Personally I have always avoided any promotion beyond SL, and regard that as, rather than languishing, providing a healthy distance from toxic politics, enabling me to concentrate as far as possible on the things I still enjoy about the job, ie teaching, mentoring, research, conferences - anything where I get to engage with the actual content of my subject, rather than all the rubbish that surrounds UK higher education today. If you're going to be marginalised anyway, it's better to be marginalised but happy. Good luck and go for it!