I was an only child until I was 11.5 years. I wished I had a brother and felt a bit lonely but I had a lot of attention and care, privacy, peace and freedom to do what I wanted. My friend had a sister she had to drag along everywhere she went which was awful for her.
I learned how to amuse myself, depend on my own resources and be independent...all of which has been invaluable in my life. I have a love of books and like my own company.
I also was around adults a great deal of the time, which meant I conversed well and had an interest in things of the world more so than my peers.
My half siblings came along later. I learned how to care for a baby, their needs and development etc. which helped when I had my own babies. But...that's when I lost my mother in effect, and my childhood ended. She was consumed by them and I was treated as the home help, the babysitter and told to grow up, no more care and attention for me any more. It was a blow. I just spent as many hours as I could away from home...with friends and their families so didn't bond properly with my half siblings anyway.
So having a brother or a sister, especially with a large age gap, can cause a rifts and problems. Not all siblings have good relationships, far from it and the tension in a lot of households can make life difficult and unpleasant a lot of the time. I've seen it.
From what you say OP you are making an excellent job of making your child feel wanted, loved and secure with lots of outside stimulus. You're doing a great job. A lot of kids like being the only child, they thrive.
The only downside for me was pressure to do well in school, exams etc. I would have done far better if I had not been forced into a school which didn't suit me but I was put there to 'look good' for the family.