I had a very stable job already that I was perfectly happy to stay at. This meant that if I interviewed at other places I could negotiate from a position of extreme strength.

So I did exactly that. I interviewed at places and when I reached the offer phase I asked for a lot. The first offer I got was not good enough, so I didn't accept it. The next one I was able to convince them to let me work four days. It was written in my offer letter and everything. I signed it, and have been working 32 hour weeks since July.

Not OP but I’d recommend looking at the r/FIRE, r/LeanFIRE, r/ChubbyFIRE, and r/FatFIRE as starting points for how people in various situations retire early, often by scaling from 5 workdays to 0 workdays over time rather than abruptly in the traditional retirement context.