I haven’t yet read full thread, but wanted to just mention a few things, from my experience.
It sounds like there are certain things that are reinforcing your feelings about this. I’m in a slightly different situation to you (and my family is now older) - but my kids have never had sleepovers with cousins (I moved away from my home town, and I started my family in my mid-30s, my older siblings all had children in their early 20s, so my kids’ cousins are all around 20 years older than my kids, and live around 100 miles away!).
We have no close family near to us, so we have built up local networks, and my kids have had lots of sleepovers over the years, with friends. (In fact, we have had two sleepovers (with different friends) this week!).
I suppose what I am trying to say is that you really can’t anticipate what your daughter’s circumstances might be when she grows and has a family of her own, she may live a long way away from her extended family, so it may all be a moot point… the majority of people in my networks locally all live a long way away from family, so have established close ties with friends. So the lack of cousins, or a small extended family is not a problem, and shouldn’t make you feel at all bad about having a single child, if that is what will be (either if you stay with your dh or separate).
I have a larger gap between me and my older siblings, 6 & 8 years, and they both moved out of the home when I was about 11, so I ended up sort of being an ‘only child’ anyway for about half of my childhood. They didn’t have too much to do with me when I was younger than 11 either, really (I think I was just the annoying little kid getting in their way!). So the family you imagine (or hope for) may not be what would happen in reality anyway.
My youngest is 12, (my other dc are much older), and I think probably in a year or two we may start bringing one of their friends away with us on holiday for company - as my youngest will in essence become an ‘only child’ very soon..
So try not to let these types of things influence your decision - as others have said, it really boils down to whether you want to stay with your dh or not (and whether you want to keep your family intact, or not - and what exactly is your priority) as there are certainly no guarantees about onward relationships / pregnancy.