Because you're very understandably completely overwhelmed at the moment, I would prioritise some personal counselling just for yourself. Giving yourself that time with a professional would help you identify your feelings, think about what future action you will take, but it can happen in a considered rather than an emotional way.
Because you're the main breadwinner and so high earning, that does present issues if you do divorce but if you're so unhappy then maybe it's a price worth paying. On the other hand, could your marriage be saved with some fundamental changes in the dynamics at home? Another consideration is the effect of a divorce on your unhappy child - would the improved atmosphere at home afterwards be beneficial for him/her or would it add to their distress? So much to consider OP. But unpicking it all with a counsellor would be helpful, I think.
Regarding your job, in your position I would be reluctant to go nuclear and leave, but maybe put out feelers at work as to whether some slight changes in your working hours/availability could be accommodated. I don't think you've said what your profession is, but I do have experience of lawyers/barristers and I know the culture there makes this something very hard to discuss. Only you know what your work culture is, and whether you could find a better work/life balance within your profession.
Regarding the house, you could absolutely downshift I would have thought. For the children to have a bedroom each and a garden you don't need a mansion. A nice three bedroom detached or semi would be fine.
I'm over 70 now - I imploded my life over 30 years ago - of necessity. I realised/discovered some pretty unsavoury things about my husband due to his lying and other behaviours. We had a lovely big home and he had a good job too. I worked in the same area as him, but left work as I had a young child - which was more common then. So I decided to divorce with a three year old, no job and unsure what the future held. It was the best thing I've ever done.
My parents were wholly supportive and helped me so much. I lived with them for a while, then got myself back in the workplace with a good job and rented the cottage next door to them from my brother. My child had security and stability of the whole extended family around but my ex failed totally as a father and bailed out. I met my present husband and he actively parented my child and gave her security and stability. She now has a great husband and family of her own, which gives us great joy.
It's not about me - but I just hope that my story can help you find a way ahead that works for you. It's never easy, that step into the unknown - but sometimes it has to be done. Change can be good if it's approached in the right way - it doesn't have to be implosion. Sincere all good wishes to you.