OP I will give you a bit of a different perspective as an only child myself.
I have one DC and we’re on the fence about having a second at all, simply because both me and DH are only children and we know the many, many benefits of that.
I will list some of the benefits as I feel MN is heavily skewed towards 2 DCs and I rarely see the positives of only children. All parents want what’s best for their children so hopefully if you read my comment it will make you feel a bit better that you will be giving your one DC a lovely life.
We are incredibly close to our parents which we haven’t really witnessed with our friends with siblings (obv people can still be close, just haven’t witnessed it being exactly like in our case). We spend all of our holidays together and it’s so cosy and fun, we all get along as I feel that our parents having only us meant they had time to really get to know us, to enjoy us as people more and both me and DH consider our parents part of our friends network. We share hobbies with them, we have great chats about literature, cinema etc. We help each other a lot without feeling guilty (I know my darling GM always made it a point to help all her DC equally but still one of her DILs was ungrateful which caused DM lots of pain), we get lots of solo time as a couple too because two sets of GP help out.
We’ve traveled loads with our parents which means we have a great love of the arts, culture etc. Our parents weren’t rich by any means but because of only having 1 they gave us opportunities that we definitely wouldn’t have had otherwise (for example I have two uni degrees, I’ve had the opportunity to explore lots of sports and hobbies, one of which became my career which fulfils me incredibly and two are still my main hobbies). My DH and I have a very special relationship which I owe partly to the fact that we don’t have siblings to rely on for that, and because of this we are each other’s number 1 person.
Of course there are many benefits to having siblings and I don’t want to take away from that but just wanted to give you a perspective from the possible future of your one DC. in