I came at this from the other side. Grew up with a sibling, always thought I would have two children. But nature had other plans...I miscarried dd's only sibling and never managed to have another one. That was difficult to come to terms with at first, but now I wouldn't have it any other way.
There are pros and cons, of course. But I think having one is amazing. To answer your specific points:
I will not be as close with my dd as I am now
I think this is almost certainly true in most cases, though others probably won't want to acknowledge it. It's certainly true of the families I know with only children. I do think the quality of the relationship that only children have with their parents tends to be quite different. There can be much more of an "us and them" thing in families with more than one child - more of a separation between the adults and the kids. 17yo DD always says that she's much closer to me than any of her friends are to their parents, and the only one who has a similarly close relationship is another only child. Of course, there are no guarantees though - only children can be distant from their parents too.
dd will lose her lovely personality because she’s not getting all my love and attention
I don't think her personality would change much, but sibling rivalry probably would come into play in one way or another, even if they got on really well. I have spent my whole life trying to learn not to compare myself to my sister, and I still haven't quite cracked it yet! I still feel unbelievably upset if it is my perception that my DM is favouring my dsis in any way, even though she has always tried to be ultra fair. My DF has always favoured me, and I know that has been hard for my dsis.
2 children won’t get on
They might get on like a house on fire and immensely enrich each others lives, or they might hate each other and make each other really miserable. You won't be able to control that either way. Most siblings are somewhere in the middle of these two extremes. I love my dsis - we played together a lot when we were kids and we get on pretty well as adults now, but she drives me crazy like nobody else! We don't live near and I don't see her much, so mostly her impact on my life is pretty neutral.
i won’t be her favourite person anymore, I will just be ‘mum’
You might still be her favourite person for a while, but that will change as she gets older in any case.
a second child might be unhappy because they also don’t get as much attention
I was the second child. I wasn't unhappy with the amount of attention I got, as it was all I knew. I definitely grew up in the shadow of my very successful sister though. Whatever I achieved - and I achieved quite a lot - never felt like much because she had already done it before me.
how will I make absolutely sure everyone gets attention and feel loved
You will be able to ensure that everyone feels loved. I don't know how, but it's definitely possible. It is a fact that each of your children will get less of your attention. Some will say that doesn't matter because they gain more from having each other. Personally, I think it does impact on their confidence and self esteem. Most of the only children I know are exceptionally emotionally secure. But of course, that's just anecdotal and there will be plenty who aren't.
what if it changes our dynamic for the worse??
The dynamic between you and your child will keep changing anyway. It's never static. But I guess it's worth thinking about the potential impact on your family if your second child had a disability or significant health needs etc. That would have a huge impact on your first dc's life.
what if I prefer new child to dd?That would be awful
Unlikely, I reckon. Love isn't finite and you would find enough for both dc.
Not enough time for each child?
Depends how much time is "enough". You'd have less time for each child, yes. And probably less time for yourself and your career. It's a question of whether you think that time and attention is important or whether having a second dc is more important. Another point to keep in mind is that there is less money as well if you have more than one dc. I love being able to afford all of the opportunities that we're able to give our dd.
dd is so lovely, how great would it be to meet another like her
Absolutely. Though, of course, there are no guarantees that your second dd would be equally lovely!! There is a huge element of luck, I think!
dd will have a play mate and someone that has her back which I didn’t have (I was terribly bullied)
Yes, sibling relationships are wonderful when they work. DSis and I played for hours when we were little and I have lovely memories of those times. My mum occasionally tried to join in, and frankly, she just wasn't as good at playing as my sister was! Having said that, I was also bullied and DSis didn't have my back at all... not interested, didn't want to get involved. Whereas my only dd has never been bullied but has an army of friends who would look out for her if she was. And she was never short of playmates tbh.
when we (dh and myself) die, she will have someone to share the memories with
I think that's true, and if I'm honest, I do worry about bit about that for dd when we're gone. I can only hope that, by that time, she'll have built her own lovely family and support network, and that she won't feel alone. I used to worry about her having to deal with ageing parents by herself, but I stopped worrying about that when I realised that I am having to deal with that alone anyway, despite having a sibling...I think I would actually find it easier if there was nobody to resent for not stepping up properly!
Ultimately, I think it's a really personal decision. There is no right or wrong, and it just boils down to whether you actually want another baby. There are pros and cons to each family set-up, and there are also so many other variables - this means that the experiences of only children and siblings are all so different that it's impossible to really generalise much.