I am so sorry that you are married to a man like this OP. I bet it just sort of snuck up on you.
LIke others, I actually don't believe this relationship can last. I am sorry. I'm not saying LTB, but I am saying that I suspect in 5 years it will be over anyway. You'll have given up because of his uselessness or, sadly, he'll have decided that you make his life too difficult and get his head turned. I'm sorry, that's an awful thing to say to someone but i believe it 100%. s
First, you have to even out finances. It's INSANE that he won't accept that the pot is smaller when you're on maternity leave. I think the only way aroudn this is to make it clear that as you are meeting too many of the household bills you will no longer be contributing to his bills in anyway. Perhaps invoice him for food and childcare while you're on maternity leave etc. Obviously, he will refuse to pay, but perhaps it will bring clarity.
I did some research and calls about finances post divorce and my understanding is that I might be worse off as he might get a higher share of the house equity as a lower earner if he wants to have 50/50 custody. And frankly not seeing DCs half of the time would break my heart.
Do you reall yhtink he'll be having the DC 50/50 of the time? Not a chance. Especially if that means he has to pay for 50% of the childcare costs that will be necessary.
Also, the finance thing is a sunken cost fallacy. even if you DO have to give him more, the sooner the relationship ends, the sooner you have time to build up your assets and earnings again, without having to share them with him.