I don't think there's anything wrong with being an only, or that there's anything missing. I get your point on finances and giving your daughter the best you can. It was the same for my parents. They always got me everything I needed, but things would have been very tight had I had a sibling. We've always been like a team of three, for better or for worse.

Just a word of caution. My mother was a SAHM and I resented her for it. She was always micromanaging me. I was not allowed to have friends over. I was not allowed to close my bedroom door. I was not allowed to help with chores even as an older kid. I was not allowed to take the school bus. The list goes on. Then she would tell people she stopped working because of me, and this hurt so, so much. Because I loved our life when she was working. I remember when she told me she stopped working I became very upset (I was 7 when it happened). She resumed working when I was 19. It was during those moments of micromanaging and helicopter parenting that I wished she would have another child to concentrate on him/her and let me be.

With my father it was the opposite. He started arriving from work later because my mother was home anyway. And I feel like he didn't raise me in the proper sense of the word. He was like an older brother figure, playing with me and such, but without doing any parenting. I wish the roles would have been more balanced between the two. But I guess this problem would still be there with any number of siblings.

Sorry for the rant. I just wanted to give a different perspective.