Well, there are very few careers that pay as much as being quant and all of them are going to be somehow similar. So lets be honest, you either get over it or take paycut be it permanent by switching careers or temporal as it is going to take time to build new consulting service or business (anything else is hardly going to pay your current salary) and you might never be able to get back to amount of money you are making right now.
Sounds like you hate it despite the pay. Retiring early is a nice idea but maybe being happy now would be worth working longer.
Have you talked to your partner? "... I can't expect my partner to move back with me..." Maybe he's more burned out then you think.
I have no idea how you spend your money from your post but maybe take a long look at your life style. Try to figure out what makes you happy and spend time on it. The less you spend the less you need to make to retire early. You may have heard this all before, but I like this post. https://www.mrmoneymustache.com/2012/01/13/the-shockingly-si...
As an aside, I've recently (re-)discovered I enjoy manual labor so have been doing yard work more. Something about it is meditative. The tasks take a long time but are simple and don't require thinking. Looks good when it is done and feels extra rewarding to actually have to sweat to get it done.
Google doesn't have only people working on ads. You could be working on new DB, OS, infra etc. I myself resigned from job for Google (not at Google) because in my location they have only crappy projects but I think in London there might be something interesting. I think Facebook might start hiring remotely (and then I might consider joining).
I personally decided to work for startup with equity as for me it is most interesting work. I also switched to investing and enjoy this extra skillset and income and maybe you might too.
Also you said your partner is making 10x, so why do you even work? My wife makes 10x less and she works mostly for fun because it's marginal change to our way of living. Don't sell yourself if you don't have to. Why are you so obsessed on salary you probably don't need instead of living best of life? You are already old (sorry)
The key part of his suggestion was “remote”. As someone else mentioned, you’re net positive comparing a high salary, living in London, with a lower paying job, living elsewhere. If you live somewhere else, you could do with less income in a more fulfilling job.
But you need to quit. You’re on a collision course with a burn out. Avoid at all costs. This will mentally scar you for the rest of your career. It’s no joke.
The important part is to get control back over your life. Your happiness.
My advice would be to jot down all your options and how they relate to each other and then go over them _with_ your partner. Maybe have them read your Ask HN post and everyone’s reply if you haven’t shared your doubts / feelings yet. If your partner wants you to be happy, and you want to be happy together, this is a problem the both of you need to solve.