Hi OP, if you don't need the job for money then why are you so worried about quitting? Do you feel you don't have value without a job? Is it just having routine that you want?
I can assure you that you have huge value with or without a job, you are so important to your partner, his mum, your children and grandchildren. Not having this particular job, or any job, won't change that.
In your shoes, if you want a job for routine and using your brain - I would quit your current job ASAP, and then give myself a breathing space of 3 months, to unpack your things, sort the house, support your partner and arrange to meet each of your children. After that, you'll be in a much better place, much less stressed and able to see the bigger picture. You'll then know whether you're ready to retire, want to volunteer, or will have a clearer picture of the sort of part-time work you want to do.
Have you considered acting as a remote consultant in your industry? My company work with these all the time - people who work solely from home on project-based work, they set the days they are willing to work and they just get paid for those days.
Best of luck, whatever you decide but please do take a break for yourself and your health (both mental and physical).