Super interesting scenario. I am far younger but recently gave up a top 5% high earner career, full payscale set out. Non retirement attrition almost zero. I am in a junior role closer to your line of work now, far riskier than where I was.
Most roles over 100-150k are rarely a low stress, 35 hour week. Even if it was, partners in a firm tend to be the type who will work 65 hours anyway. You say you like the job - so you almost certainly sit in that wired for work camp, and many of us fall out of it when the health scare eventually arrives.
Day rate consulting, 3 days a week, seems the obvious route, you perhaps have a skill/expertise that lends itself to doing this? Beyond that or some board roles, I am not sure this 100k, low stress, job you speak of exists? If you have identified the role on paper, would you operate to that role definition, or just continue your current behaviour/ passion/ work ethic of going above and beyond?
On the financials, perhaps a counter view; regardless of wealth - extremes excludes - discussing upsizing house while cutting salary by 50-75% is rarely going to be comfortable. I see an uncomfortable trap here. Giving up 400k role, to make yourself uncomfortable at a later stage in working life is relatively risky business. Fine if you're 30 and have time to dig out of a tight spot or bad luck.
With ~30k average a month I'm not sure why your pension pot isn't full? Or why there isn't more savings. It probably does not change the dial but are you aware of your outgoings- are they under control? If one isn't disciplined on this front an income drop will be all the more painful.
Are you tied to your current location due family or attachment? I think that influences options significantly. Move out to a mansion outside the m50, work 3 days of week. Otherwise do you really need a bigger house if you got this far with the kids (who are maybe more than half way to moving out!)
Going back to the fear of the unknown, if physical or mental health is at stake then I think you must protect that first. That means your partnership facilitate your needs or you walk away. Your daughters will want you alive, not sick or dead but in a bigger house. I eventually made the decision to take a leap when I came to terms with the fact myself and my kids would not starve, even if plan b or c didn't work.