The arrangements that hold up are the ones where the difficult conversations happened before conception rather than after. Here are the questions worth answering together, in writing, while everything is still hypothetical.
The child’s daily life
Where do they live, and on what pattern? Who does school drop-off? Which language is spoken at home? Which school, and who decides? What happens when the answer changes?
Money
Who pays for what, in the first year and in the fifteenth? What happens if one of you loses your income? Is there anything paid between the adults, and for what exactly? Who insures the child? What about university?
Belief and upbringing
Religion, if any, and whose. Diet. Screen time. Discipline. These sound trivial next to the legal questions and are the ones that generate friction weekly rather than once.
Health
Who decides on medical care, and who can consent in an emergency? What is the family medical history on both sides, written down? What happens if the child has a serious illness or disability — and have you both actually said your answer out loud?
The unwelcome ones
What happens if one of you dies? Who becomes the child’s guardian? What if one of you wants to move — to another city, to another country? What if one of you has another child, with someone else? What if one of you wants to stop being involved?
The donor’s place
Contact: how much, how often, and who decides? What is he called? Does he attend birthdays? What do the wider families know? What happens if he wants more contact later — or less?
How to use this
Answer separately first, then compare. The value is not in agreeing; it is in finding out precisely where you do not, at the point when that discovery is cheap.