Would it be possible for you to take a career break for, say, 6 months to a year, OP? Because it might be foolish to properly give up your job, and find a few months down the line that you hate SAH.
Another option would be to look at working part time, so you have a couple of days a week when you are SAH.
Personally, I think it is possible to get yourself organised to the point when you don't feel like you're spinning plates and all is constant madness. DH and I both work full-time. I have a cleaner and shop online, which is lovely as far as I'm concerned since I hate shopping and cleaning. Are you sure you'd really want to do those things yourself after the novelty has worn off?! We make sure that the DDs only do a limited number of activities, and try to co-ordinate activities around each other so we're not running in several directions at once. We split the school runs and collecting, we split the housework, we pick up for each other when one of us needs to go away to a conference or work late.
I don't feel like I'm spinning plates. The house may be a bit scruffy between cleaner visits but who cares? The DDs are in childcare a lot but they love it. I go out a couple of evenings a week to my own activities, weekend afternoons are very relaxed. I grow vegetables, go running, the kids go over to friends and have friends over.
I would suggest you sit down, look at all the things you're doing, rank them in order of importance and see if you could simply do less. Quite often, I think, people take on more stuff than they need to and are so busy rushing that they don't realise not all of it is vital.
Good luck whatever you decide.