@TempsPerdu (and others facing the same dilemma)
If you want to stop at one please have the courage of your convictions to do so. I did and I am so, so, so glad I did.
I’m a mum of one and I absolutely love it. Only have to focus on DD, can plan life / weekends / holidays around her needs & wants - no need for her to miss out on activities because of money or because a certain activity is not suitable for a younger sibling.
She’ll be going to private school & while we don’t spoil her in terms of material possessions, she’ll be able to pick & choose her hobbies, including expensive ones if she wishes! We can fund university & help her onto the property ladder.
Most importantly though, I just have so much time for her, I’m not pulled in different directions.
I also have time for me (as does DH) and am heavily involved in a sporting hobby which is amazing for my physical & mental health. It’s also easy to get a babysitter for one well behaved child.
DD has been in nursery part time since a year old & is very sociable and kind. Another parent recently complemented me on what a lovely child she is. All that “spoilt lonely only” stuff is utter rubbish. (My own best friend is an only child & is one of the most stable, independent, interesting, awesome people I know).
On the otherhand I have 2 siblings & we are not close at all. Unfortunately one of my siblings has some severe mental health issues which have seriously impacted my own relationship with my mother in recent years as my sibling is so dependent on her
It’s the source of much sadness & worry in my life.
I also have a friend who went on to have child number 2 even though she’d struggled hugely with no.1 & she’s now completely and utterly overwhelmed and struggling to cope 
I also very sadly have a friend who had a severely disabled last child (at about age 40 although not sure if her age was a factor) which has had a huge impact on her & her existing children & turned their life upside down.
If you want another child then have one. But please don’t bring another human into the world to provide a sibling 