Garrett, I add my voice to everyone else saying don't do it, and probably this relationship has come to an end. You don't want the same things, you don't want to live the same way. The only circumstances in which you would go along with the way she wants to live is if EVERYTHING works out the way you want it to. It doesn't have anything to do with finances or FIRE. You do not want to take chances. You just can't get that in life, but you can get close by not having children, not allowing someone else a say in your financial life, by working toward FIRE so you will be secure not matter what. You say she is a great person, but ultimately, you do not trust her or you wouldn't be on the internet looking for answers.
You have to have an honest, MATURE conversation with yourself and with her. Start with, what if everything that could go wrong, goes wrong? What if it's twins? What if the child is disabled? What if she wants to split, but you don't? What if she runs off with the postman? What if you want to split and she doesn't? What if she wants more children and you don't? I will give you a big clue: if a woman wants a baby, she is gonna get pregnant (other than infertility). It is not up to the man. Women have been getting pregnant to get what they want since the world began. And I say that as a woman. How about this one: what if she wants to split and doesn't want the child full time or part time? It happens. I am the stepmother of two children whose mother split.