Is the CMS shit? Yes
Should a parent be able to walk away and not have to pay half the costs of raising a child? No
I think most of us agree that CMS is wildly out of date and not fit for purpose. OP, I'm sorry you've been left with all the practical responsibilities - it really is unfair.
However, I do think it's worth looking at the actual figures we're talking about here because they're quite surprising when you look at net income.
I'm assuming no contribution to pensions, this is just the basic income:
Ex (OP says nearly £80,000 but I've rounded it up to £80,000)
Take home salary: £4683
Minus CMS of £730
Net income = £3953 per month
OP salary - £46,000
Take home salary: £2998
Plus £730 CMS
Plus £400 UC (multiple PP have checked and found OP is entitled to £400)
Plus Child Benefit £100
Net income = £4228 per month
So OP will actually have £275 MORE per month than her ex when all income and deductions are factored in.
Of course, currently OP has huge childcare costs to deduct from this. But if she claims the free hours, this should cut a fair chunk off the cost. Plus there is the option to look around for cheaper childcare. No, it's not ideal but it's an option - even if it's just to go on a childminder's waiting list for September or January.
When the child is old enough to qualify for 30 hours free childcare, the costs will reduce further.
And then when the child starts school, the childcare costs will drop even further meaning that OP could end up on almost the same income as her ex, assuming no changes in their salary.
OP has said she doesn't mind having to cut her cloth according to her new income but doesn't see why she should have to when her ex is paying very little, in her opinion. While I agree that the CMS is crappy, I'm not seeing this as terrible.
It's mind-blowing really as it sounds as if there's a huge disparity between a salary of £46,000 and £80,000 but surprisingly, that's not the case - especially when you factor in the CMS and UC. They're almost on level footing.
The big expense here is the childcare - if this can be reduced with the free childcare hours/UC childcare help, or a cheaper provision found, that would make a big difference to OP.