If I were you, I would agree to drop your claim to the child benefits if he agrees not to pursue you for maintenance.
I know it’s very frustrating that for years you did everything and sacrificed a lot to raise the kids and now he’s swooping in and taking more time once the hard years are done. BUT the courts won’t concern themselves with historical splits because child benefit should go to the CURRENT main care (which you accept, albeit by a small amount) is your ex. And CMS should be paid by the parent who has them less to the parent who has them more (you accept right now, you have them less and he has them more).
This isn’t your money - child benefit and maintenance payments are to help pay for the children. The person paying for the children more right now (and maybe since March) is him. Therefore, he deserves the payment.
I get money might be tight and it’s hard for everyone at the moment, but I think you need to realise you’re not entitled to this money because it’s for children you’re not currently having as much as he is…
If you fight for this money, you anger him more and he will potentially open up a maintenace claim against you with CMS or have the children more and more. You already cut your nose off to spite your face by rocking the boat when he was generously giving you more than you deserved, so stop rocking the boat, accept defeat and start to figure out ways to make some extra money and repair the relationship with your children. The fact your eldest now spends 80% of their time at your ex’s house speaks volumes as to how this conflict is affecting them.
Please just listen to what literally everyone here is telling you, you’re becoming so fixated on this £70 per month or whatever it is that you’re losing perspective.