I spotted your question on the Victoria Coren thread OP. You sound lovely and thoughtful, and are clearly going into this with your eyes open. There are lots of wonderful things about having one child (if that does turn out to be what happens with you), and on paper it’s still very much the logical, pragmatic choice for our family.
But there are also some potential downsides, and they aren’t necessarily the ones DP and I had agonised about before making our decision! I have my own thread running in this forum, and while there a lots of other factors contributing to our particular scenario I do think the only child thing feeds into DD’s difficulties in establishing a social network.
We were 37 (me) and 44 (DP) when we had DD. I’m not an only myself, but my only sibling moved overseas, and my parents were also older so I grew up separately from my much older cousins. Very small family generally. Now both sets of grandparents are in their late 70s/80s, so we had to factor this into our decision making process - part of the reason we ended up with one was that we knew we couldn’t expect any help with childcare. We’re very self sufficient, but I must admit it’s been tough at times having four grandparents who are literally just up the road but unable to give any practical support (although to some extent this is a temperament/willingness issue as well as a frailty one) - we are very much a ‘tag team’ at times and rarely have an evening out or any time alone as a couple. And of course we’ve now perpetuated the cycle by having DD later so we’ll be older grandparents too! Sounds like you’re in a better position with that though, and tbh I’m mainly just grateful that her grandparents are all still alive to see DD (3 of mine had died by the time I was 5 so I never had those relationships).
The advice pps have offered so far is good, but I would say to make sure you’re in a position/location where your future DC is not the ‘odd one out’ and where they have the best possible chance of forging their own social connections, not just hanging on the coattails of your/your DP’s existing ones.
This is something we were a bit naive about before having DD. We have done literally everything on @JustAMinutePleass’s list - all sound advice, but for us it hasn’t worked because where we are our lifestyle is just so different from ‘the norm’ that DD remains an outlier. And, if you read my thread and other discussions on here about only children, doing all of these things leaves you vulnerable to accusations of being ‘over invested’ in your child. I think sometimes those who do have strong extended family and multiple DC just don’t get the extra effort you have to make to socialise and forge a network for your child, and can see all the chasing play dates and volunteering as being a bit odd and desperate.
Locally, we only know one other one-child family (from our NCT group, and not an only by choice). DD is the only one in her school class. People are also extremely time poor - partly because two FT working parents is the norm, partly because no one ‘comes from’ where we live, so everyone disappears off to visit family and friends during school holidays, and partly because everyone else has multiple DC to wrangle. So play dates are very hard to pin down, and holidays can be exhausting as we have to think hard and plan lots of days out and fun, child-friendly activities rather than the ‘just go and play in the garden with your brother’ or ‘let’s drive over to see your cousins’ that is typical for DD’s peers. I think there’s an element, too, of people having retreated into their families after the pandemic, with fewer people feeling like they want or need a wider network of contacts.
Luckily I’m quite child oriented myself (professional interest in child development) and enjoy (to a point!) playing with DD - I’ve done hours and hours of role play in my time! But it’s tiring, and I think this might be quite tough for others who are less naturally inclined!
Basically I’d say do it, don’t feel selfish - it isn’t, any more than any other family planning decision - but do lots and lots of planning: think carefully about your location, social network and DC’s future school, plan ahead for old age to mitigate problems further down the line, and, yes, follow all of @JustAMinutePleass’s excellent advice, but accept that your efforts won’t always be reciprocated.