My heart wants a 3rd child, my head doesn't

Hi lovely people of Reddit, I am new here but desperately need advice please.

My partner and I are in our late thirties and we had spontaneous identical twin boys 5 years ago. It took me a while to agree with the idea that I have been lucky with the fact that I had twins, mostly because the past 5y have been full on emotionally, financially and on other aspects. My partner and I are nevertheless strong and things are easier now that we're out of that shi**y toddler phase. They are amazing little men, they crack me up and I can't wait to see how they grow up.

I have however always (since twins) craved the single child experience and possibly having a girl after two boys (but of course I can't choose and that's fine). So I have been thinking for a while about having another baby, and I can't seem to get this out of my head. I am myself my mum's baby girl she wanted so much after she had two boys (my two older brothers - not twins).

My partner fully supports my wish, he says he doesn't want me to have any regrets later on.

Now for the problem: following his dad's passing, we recently learnt that my partner is a carrier for a genetic trait from his mum's side that increases the risk of cancer (mostly prostate for men and breast for women). It's particularly severe for women. As he actually lost his mum when he was a kid because of this, it's not an option for him to conceive this third child naturally (we had the twins before we learnt about this) so our only option would be IVF, which we are seriously thinking about. Doing IVF would be very expensive but we have a way of getting the money (legally lol) and we have both made peace with this aspect. Aside from this, and since this decision will fall on me, I am finding myself completely torn. I want this child but I don't want to compromise on our happiness. I want another baby but what if I get an annoying child. What if I get a sick child. What If I have twins again!!!! I Does anyone ever regret having that third kid? I am convinced I was destined to have kids. I love my boys so much, they fascinate me, they make me laugh, but they're also an everyday challenge because having kids is hard work.

I also love going to work and be independent. I would not want to be a stay at home mum. I also think our logistics as a family would be tricky. Would we need a bigger house? We don't have the money for that. Do we need a bigger car? Would we be ok with paying huge nursery costs for another 4years as we just finally got out of it?
Would I keep my sanity going through the lack of sleep again? That shi**y toddler phase? The tantrums? My twins are already disputing my attention every day, how do I divide my attention with another kid on top?!

My partner is less stressed than me. Or at least he doesn't tell me if he's worried. He just wants me happy and if I tell him about my worries, he remains very pragmatic and tells me the good sides of having a baby. He thinks it will be good for the twins to have a sibling. In a way, I feel like it would have been easier is my partner had said no from the start. Then I would have stopped that idea before It goes too far in my head.

Also my body isn't as young as it used to be, I can feel it. My back hurts often, I don't loose weight as easily as I used to, I catch all my kids' viruses. Would I put my health in jeopardy by having another one??

I am just so lost. Everything in my head is telling me that my family is ok as it is and I shouldnt compromise it. But I cant stop thinking about it. It feels like my own little selfish tantrum. I see pictures and videos of my twins as babies with emotion and everything feels so blurry in my head. I enjoyed the newborn and small baby phase SO MUCH and I want to live this again, but with only one single baby. Well I obviously forgot the bad moments. It's human nature. I am 99% sure I will regret it if we don't go ahead with it.

I am determined to take the decision by next summer so I get pregnant by end of year and I have that baby (or not) before I turn 40.

Strangers of reddit, please help me put things into perspective, bring me your own experience, and help me be confident about a decision. Thank you for reading and sharing your wisdom