I really hope your ‘partner’ does read this, OP.
What you have described is a system that a lot of couples end up using. But it is still a shit system. The woman/mother ends up being penalized for carrying carrying and caring for a child.
I think it is outrageous that people think this is ‘fair’. Creating a child together, and parenting one, is far more difficult that managing finances. If you are prepared to do one, then do both, FFS! And do it properly.
Let’s be clear - that child is 50/50 yours and his. He needs to be, no, he is responsible for 50% of everything. EVERYTHING! Night wakings, nappy changes, childcare drop offs and pick ups and birthday parties and presents and food and clothes and furniture and everything!
Now, let’s be realistic. In a couple, it can make sense for one person to take the lead rather than everything being 50/50. That is particularly true when it comes to parenting but can also apply to housework, cooking and car maintenance. The reasons for this are varied - time, inclination, and ability.
So one part of the couple can, by agreement, takeover more responsibly for one thing but only if the other part of the couple takes more responsibility for another.
With babies, that trade off is usually between childcare and money.
Your ‘partner’ needs to prove that the two of you are really going to be in 50/50 partnership. Calculate your costs (including the breast pump that feeds his child!). Your partner is responsible for 50% of those costs. Calculate your losses (the weeks of half pay, statutory pay, no pay). Your partner is responsible for 50% of those costs.
If he will not cover these expenses, then you will have to go back to work earlier than planned. And then you both will be responsible for 50% of the child are costs. No, nursery fees don’t just get paid by the mother. They are the responsibility of both parents. And it may well be financially worthwhile to both of you for one of you to stay at home with the baby.
Of course, if he doesn’t agree that it is financially worthwhile, he may want to stay at home with the baby himself. And that is his prerogative. And he can take that career hit. Except you said he didn’t want to.
Alternatively, you can both enter into an adult relationship and pool all of your resources for the benefit of your child and family.
PS - if you don’t want to get married, that is fine. But please note that there is fuck ton of paperwork you should take care of now you have child. Do you have wills? Do you know who the beneficiaries are? Is it sensible to leave everything to a child who cannot access it until they are 18 or should it be left to the guardian of the child to spend on the child as they see fit? (Surviving families have been left homeless because the child’s money was tied up and couldn’t be used to pay for accommodation for the surviving parent and child). Who makes decisions if you are medically incapacities? Is that written down somewhere or with parents try to intervene? Is property held as joint tenants or tenants common? If you split, will you share savings that have been made due to one person covering costs that may have shared? The list goes on and on and on. Getting married is actually easier.