I have a 18 month old and really want another but I am already in my 30s and would either want another in the next couple of years or not at all. Unfortunately my DH is chronically ill and so it's looking like that wont happen. I always imagined I'd have either no or several children and I've had similar thoughts to you. However...
People often speak as if there are two options:
a) people are close to their siblings and being an only child would have been terrible
b) people don't get along with their siblings and don't speak or argue and being an only would be better
I'd like to mention a third option:
c) I'm close with my siblings but, thinking coldly and logically, still think id have perhaps been happier an only child
I have two older brothers and we got along surprisingly well growing up. We didn't randomly kick each other like my friends siblings and we didn't argue much. We did spend a lot of time playing together. However my brothers were always very close to each other and I always felt like a spare part on the outside.
In hindsight, especially since I was the youngest, they were very patient and spent a lot of time with me but I was still desperately lonely. They were "the boys". They were best friends. They weren't twins but they were treated like people often treat twins. I so badly wanted to be part of that.
I have memories of being left out in games or not being allowed to go out with them when they went off exploring on their own (because I was too young). They were part of one big friendship group - a load of boys from our street - and there weren't many kids my age.
I'd cling on to any bone they threw me and I just wanted nothing more than to be "one of them". From an early age I was acutely aware of my role of "annoying younger sister". Not that they were horrible to me. Just that hanging out with me was more like babysitting than hanging out with a friend.
And no, we did not have a massive age gap (I'm 7 years younger than the oldest and 4 years younger than the middle). Which means they are 3 years apart. There's no reason in another sibling group with the same age gaps that the youngest and middle wouldn't be tighter knit growing up.
We had so many lovely memories and we did still spend a lot of time together but I've kept those feelings of feeling an outsider with me my whole life.
I don't think "three is a crowd and four is a party" is the answer either as lots of dynamics of three siblings would be different to ours. I don't think any of us did anything wrong and I am ND so would likely have felt different to others anyway but I do think it affected me.
Most people who dabble in psychotherapy or similar therapies find links to their childhood and their current patterns in relationships, work, friendships especially with regards to their early relationships with their parents.
However I do not think enough attention is paid to sibling relationships. They are such important relationships and we are with them all the time.
I'm reading a book that speaks about upbringing and it basically argues that you and your siblings did not have the same upbringing. There were the labels that were put on you growing up as well as expectations about birth order, gender etc.
I have to agree that I think I would be a completely different person if I'd been born the oldest for example. I also expect that if I'd been a boy things would have been different too.
Now I reflect on my various issues as an adult, I do see myself repeating patterns that come from my relationship with my parents but I also massively see similarities in my early relationship with my siblings.
For example, I'm constantly pursuing quite cliquey but tight knit friendship groups that I aim to fight my way into in a hope to feel "part" of something and to belong there and be wanted but once successful I feel uncomfortable and struggle to maintain the friendship.
It's a subtle thing and something I didn't notice about myself for years but now I see myself doing things and have these flashes of childhood memories of feeling alone and trying to be good enough and grown up enough.
Now maybe I'd have been more unhappy an only. I don't know. It's hard to truly divorce myself from it. I know and love my brothers and now we are adults we are all equally close to each other (although drifted a bit in recent years for various reasons). I would never wish for them not to exist and I do still look back mostly fondly on our years growing up together.
But if I try to be objective and logical, I do think that it's likely that without always feeling left out and having more time/attention from my parents I'd have probably turned out a bit happier.
When we introduce another sibling it's so massive and adds so many more dynamics to the mix. I do think it's probably the safer option not to do so. It's hard because you don't want them to miss out on siblings but I wouldn't assume that the default option is "siblings are better" except for extreme cases where they hate each other.