I have a 12 year old only DS. He isn't an only by choice, that's just the way it worked out.
He is happy, unspoilt, and fits in beautifully when at any of his many friends houses. He goes to a Catholic school, where only children are very rare, and he never feels like he is 'odd'.
He is lonely sometimes - he'd love to have somebody in his house to play with. I do worry about what happens when DH and I are older - I have health problems that mean that even though DH is 11 years older than me, we're likely to die at similar times. Eats away at me tbh.
There are up and down sides whichever way you go.
I can't give him a sibling, what I can do is try to give him the skills to build close relationships that will take the place of siblings as he grows older.
Only children don't have to be sad, or lonely - but there is the chance they will be. Similarly, children with siblings don't necessarily feel like they have an ally.
You have to do what you feel is right, and accept that whichever path you take may not be perfect.
FWIW - I hate the automatic assumption that only children will be sad, lonely, spoilt. Many parents of only ones try very hard to ensure that situation never happens. I'll tell you in 20 years whether DH and I got it right.