You were NTA until I read this part "I'm also going to file so I could get child support from her and her ex-husband."

If you have to give her money all the time, what makes you think she has money to give you? It sounds like you are also doing this entirely behind her back? What about the ex-husband, if he has a job and can pay you child support, maybe he would like to also parent his own children?

To be clear, because she was abusive to me, I'd give her minimal supervised contact with them.

It sounds like this is indeed revenge. I fought with my siblings and was abused by them as well when I was young, it took me years (I was much older than just 25!) to finally forgive them. They do not and would not abuse their children, the reason we fought as children does not relate at all to our relationship as adults.

I think your plan is selfish, get CPS involved if you think her children are being abused or need to be re-homed. Let CPS evaluate you and decide if you should be the guardian, maybe your parents should be the guardians?