I’m sure this is unpopular, but hear me out.
If she’s moving on and leaving you and the kids, I wouldn’t try to force a different outcome.
I paid for the ex’s phone and car insurance for a couple of years until the divorce was final. While she stayed out of our lives, that was worth it.
My point: don’t provoke her enough that she runs to court and takes the kids. Narcissists thrive on control, so slowly separate your finances and communication, piece by piece, not all at once.
Giving a narcissist even one thing to argue over only fuels them.
Set a clear date say the end of the month or the billing cycle and tell her that’s when you’re cutting her off. Then stick to it.