You are right. He should take equal responsibility. Unfortunately what is morally correct is not very often what happens.
My ex on paper is a great dad. We have 50/50, no court orders needed, he paus maintenance reliably. Not the amount the CMS suggest, but a fair amount. He tips it up with 50% of all other costs for clubs, school trips etc.
In reality he is utterly useless at life/adulting/parenting etc.
Sometimes it is about picking your battles.
In your case, you are not unreasonable on the surface. But if he is unlikely to commit to regular contact, then is tryong to forcing him to be involved perhaps counter productive? Could the repeated in and out of their life be more damaging in the long run?
As much as I fully appreciate your need for time out, in becoming a parent you choose to sacrifice your own wants and needs in favour of what is best for your children.
I'm not sure if I have missed the age of your children, but perhaps sleepovers with friends or are the older siblings of friends old enough to babysit to give you the odd evening off? Are there grandparents who are able and willing to have them overnight?