When you say you've agreed Child Maintenance, was it court ordered as part of the divorce? If so, that only lasts a year, and after that you can go with CMS assessment.
Assume there are no court orders for the Child Arrangements? It's not nice being threatened with a) taking the kids away b) threatening it'll cost you more etc.
One thing you could do is get the Child Arrangements formalised with the court at 50/50 (and in that case there is no CM to pay! Unless you make a voluntary agreement between you). However court applications can increase hostilities - the usual process is try mediation first (which probably won't work from what you've said) then apply to court (it is possible to get an order at first hearing if both agree - but again she sounds unlikely to agree to no money).
So reading between the lines what you've said. She is giving you the kids a lot - because she's working? And saying if you can't have them she'll have to give up work to look after them, is that right? While there is some sense in that, it's not how it should be. If she needs to work, she should work, but also work out a reasonable schedule with you.
I think most Dads would love to have their kids more than half the time, but I can see your point that it's hardly fair to do that and pay a large sum of CM as well. But presumably if you didn't pay any, your ex couldn't afford to house the kids is that right?
One thing you do want to do is build up a good history of time with the kids, and keep records. Write it down on a calendar or in a diary, exactly when they are with you each time, what time till what time and any particular things you do during that time. Take photos when you have days out etc. You probably do anyway, and don't be too obvious about it.
So if and when you do have to apply to court, you can prove the history and have a good standing.
How long has this situation been going on (ie you having the kids most of the time and paying a third of your salary to your ex?).
Another reason to keep records is - if it suits her she might reduce that time drastically - for various reasons - and then you would need to apply to court.
Ultimately parents have to provide for their kids - a roof over their heads, pay the bills (two homes once separated), clothe, feed them, get them toys, pay for school trips etc. How you work that between you is individual in some ways - but if you're paying twice - not good.