OP, at this stage (with him actively making plans to take you out of his will), your best financial plan will be splitting up, whether that is at your instigation or his.
Sounds like housing costs are low in your area. You just about have a deposit. Save aggressively for another 12 months, or until he decides he’s had enough if sooner. Can you get any overtime? A promotion? A second evening job? The kids will be fine for a while if you’re not around in the evenings or weekends - you have to prioritise yourself for a bit.
Then you buy yourself a small house, and with your salary, and benefit top ups, and CMS, you’ll certainly be no worse off but you’ll have some security.
You’ll also have to base all your financial decisions on your future security, not providing for your kids, either while you are alive or after you’ve died. Their father is rich, if they want bassoon lessons, funding through higher education, or the school ski trip - they ask their father. All your money needs to go into retirement savings, you don’t need to leave them anything because their father is leaving them a fortune.
And finally - make sure your kids do know the score, especially your daughter. It sounds like you didn’t have much of a career to speak of before the kids came along. She needs to know that whether she works in Tesco or for Morgan Stanley, she needs to have her own income and own savings, and not to give either up.