To be honest his vagueness/unrealistic approach to actually parenting is very worrying. You will know yourself , in a way that you might not have when you were younger to look at who people are showing you they are, not what that they are saying. He might be a loving stepfather, but that only means he will probably be a loving father. Not that he will step up and be a fully hands on, sharing care and domestic chores fifty fifty. His comments about child care are worrying, as is his not wanting to seriously discuss a marriage ( not wedding ). Have you discussed the ( small) possibility of a child having additional needs/being disabled.
You said you were open to another child but you are indicating you’re not keen, and that’s reason enough not to have one. In fact it’s a good one. You need to consider all the possibilities including the worst case, of him not being up to it and you being on your own with three children, one a baby. Being unmarried would really put the tin lid on it.
I am not being negative, just realistic I hope. If he can’t even agree to a small ceremony, with you, your DC and a few others before a baby it’s a poor show. A big party can be held in the future if you want it. You are right, a huge wedding at this stage, with a baby planned would be a waste of money .
Please think carefully, and selfishly, we are only here once, and it’s a short timer. I met my partner( no DC )in my late thirties, was clear I was not having any more children and he accepted it as he wanted to be with me. We have been together over twenty years ( only living together the last six or so ), and he continues to be a fabulous step father. More so than their dad, who of course loves them, but not the most involved or hands on of parents. My DP seems to get a huge amount back from my DC too. If he has regrets about no biological children he has not told me, and clearly has channelled a huge amount of any such desires into loving my/our DC. It is possible.