I have an only child. She s 9y and she is in private education.
Afaik I have a very happy,sociable young child. A sibling may have been nice but we don't angst over it. It's out of our hands. Dd has sometimes asked re siblings but understands.
Our decision to use private education was unrelated to her having a sibling or not. Obviously there ony being one child does make it financially easier though.
I know several on,y child, most wadults. All are fine and happy, no social issues, no worries over older parents. I also knw some people with siblingsvwhonreally dont get on. I aso know adults with siblings who are still left being he ony one looking out for an ageing parent.
Dh is a solicitor and deals with families every day. He sees so many fall outs and nastiness between siblings, very sad.
You can't know what will happen between siblings. Often it is good, sometimes not. Have a second child fr yourself, not just as a sibling.