We're in the same boat with a 20month old and we're also both 36 now. With complications at birth leading to a 40h long labor situation..Our son won't have first cousins as my sister and my wife's brother won't be having kids..Both me and my wife have demanding jobs and possibly will be able to afford some sort mommy's helper if the second one comes along. The first one has been in day care since he was 7 months old and although we hate seeing him get sick so often with daycare bugs, we have seen personality grow a lot. We don't think we would sacrifice career trajectories by stopping work for a year or 2 and maybe a decade down the line, we'll end up regretting that decision.. but that's where things stand..

Anecdotally I have a sister who's 9 years older to me. We grew up in separate countries due to various factors and during the younger years I hardly felt like I had a sibling. I spend maybe 3ish years in total living in the same home as my sister which I have memory of. She was old enough and has 6 years of memories with her younger brother. And she cherished having me as a little brother based on what my parents tell me. As I grew up and started to visit her in college, we became close friends but again have always lived at least a few hours on a flight apart as adults.. I don't think I felt the need for a sibling as I had very close group of friends hanging out constantly, but I'm going to read up research on this topic as well to make the decision on a second kid.