Why don't you and your DH want another? You sound definite about that. How strongly do you feel about it? That reason and your strength of feeling and belief in it must be a factor in how you're likely to respond to another child.
I'm in a position of viewing the 'second child' question from a different angle. We'd like one 'for ourselves' and to create the sort of family we'd like. Certainly one other reason is to give dd a sibling.
That's not based on any assumption they'd get on but one reason is about sharing the future burden of us in our old age. If dd leaves starting a family as late as we have, we'll be in our 70s when her DCs are born. She could well be dealing with our decrepitude at the same time as her own young DCs or teenagers. Being an only child of older parents presents the prospect of a particular sort of loneliness and potentially a large burden of responsibility. (Of course having a child in my 40s holds potential to create an additional burden on dd, rather than lightening her load, which is another train of thought entirely).
I was an only, though with younger half-siblings and the relationship with them has really come into it's own in adulthood. It is good to have other people in my generation (you can't rely on there being close cousins) and, I'm sure I wouldn't see their parents so much, or do anything like as many family things, if it was just me and the older generation. I do get a lot out of that stuff and of having them out there (and we do get on, which can't be relied upon but is also partly to do with upbringing and the family dynamic, so a sense of family as being important and a source of enjoyment).
As an only I wasn't always lonely, I happily read a lot and had friends to play with. But, particularly in the teenage years, I was very aware of friends' experience of being able to take company for granted and have much more fun within their families - and with other larger families whose parents naturally befriended one another - than I did. I had to seek out and make an arrangement to have company, every time. Especially when friends are busy doing fun family-only things, that is hard.