H.M.
You have to remember that in large families, there is bound to be a large gap between the first and the last! It's perfectly fine! I only have two, but they are almost 5 years apart, and it's been great! My oldest got all my time and attention for the first 5 years of his life. Then he went to kindergarten when my daughter was 2 months old, so I had time alone with her everyday as well! Both my kids were able to get significant one on one time, which I believe is a real bonus to their early development. And while being a boy and a girl and nearly 5 years apart makes it so that they are at very different stages in their lives, it doesn't mean they aren't close. My daughter's best friends are my son's best friends' siblings, so they often hang out in the same group. And they share a love of music, my son plays guitar while my daughter sings. They have plenty in common, and yet we've never had the competition that comes when they're closer in age.
Also remember that no matter what age difference there is, each child is different and may or may not end up being close friends with their siblings. There are kids two years apart who grow up very distant from each other, and there are kids 6 years apart that grow up best friends. It just depends on their personalities and how you as a parent foster their relationship.
The average span between kids is about 3 years, so that means the average first and third kids are 6 years apart. That's perfectly normal. I think whether you choose to have more kids now should depend on the other factors you mentioned, the age span is really not an issue.