My heart goes out to you OP, you're in a tough position. I was soooo happy to read how you stood up to him. I can see how your mindset has changed through the thread.
Do you love him? And do you think you can work through this? It doesn't sound unsalvageable to me, but it does sound like you've been very unhappy and need to consider your future. Maybe along with your day off to study, try and build in some time for you to spend alone with him where you can start to rebuild your relationship?
If the answer is no, and you're both checking out emotionally, then you're doing the right things in preparing yourself to leave.
Is there any way you can build bridges with his family? The more they can see you as the loving, self sacrificing mother you are, who has facilitated DP's life and career, the more support you'll get from them. Never mind your background, you are making huge contributions to your family household. Did his DM work? Do they realise you literally have nothing for yourself?
And maybe draw up a proper budget to show him: kids clothes, activities, travel, groceries, standard things for you - like haircuts. So it's not necessarily you asking for limitless cash, it's you saying "I need x amount for routine purchases." Hopefully he'll see that you're not being unreasonable, and you've been struggling for a long time.
Someone mentioned upthread, but look at private tutoring or TEFL. You can do it freelance, online, a few hours at a time, it's simple and reasonably quick to qualify and you'll have loads of those skills already from home educating your own DC. Something like that would help you to balance your home life and start earning money for yourself pretty quickly. Could you offer English classes to your neighbours perhaps?
Re OW, I think you will need to bring it up with him at some point, because you won't be happy for that behaviour to continue. But it really depends on whether you want to save your relationship and see a future there, or if you've had enough and plan to leave.
Good luck, whatever you decide xx