@mindutopia gives an excellent steer.
@Baker123 Try to remember that just because you have an only child and you are of sound financial standing, doesn't mean that your child is showered with gifts.
The benefit to being an only child is that they have their own bedroom, accessibility to private schooling is easier as you just pay for one not two, you can have more extravagant holidays as you have fewer flights to pay for and can book a single room for the entire family, they can participate in more after school activities (sports, amateur dramatics, musical instruments), etc., they can have tutors for the 11+/GCSE/A-level, they can go to university and not have to graduate with massive debt as you can support them, you can give them a deposit for a house, you can help your grandchildren with all of the above, and they dont need to share the house they inherit when you die.
None of that relates to being able to shower your child with gifts - which actually devalues every gift you give them.
Giving her a virtual birthday in summertime because she is born in Dec/Jan works best when it is intentional and allows an outdoor celebration or trip that is not possible in winter, while preserving the magic of the birthday feeling with a celebration, cake and blowing out candles. Just simply saying that she can buy something herself doesn't achieve that and it's too early for a child to have access to significant sums of money at their disposal.
You mentioned that she has a smartphone. Have you utilised all parental controls? Does she have WhatsApp/Insta/Tiktok? Does she keep her phone in her room overnight? Only children who are girls often rely more heavily on digital relationships and influencers so are more susceptible to lookism and cyber bullying, which can affect their behaviour to their parents. It may be worth auditing this and ensuring that you keep the phone overnight.
There can be friction in a couple over differences in parenting approaches. Try to decouple this from your daughter's behaviour and agree on a parenting strategy with your husband. Then as a unified couple, implement that strategy consistently and calmly in parenting your daughter.
I would suggest reducing the focus on lavish spending and increasing the focus on engaging with her in conversation as a person and instilling in her the values that you believe in. This will help her to understand the mission of the family and to feel an integral and important part of the team.
Her body is big now but emotionally she is still the 9 year old she used to be. She has no sibling to talk to you, so that has to be you and your partner. Ferrying her off to sports training and competition every hour god sends is great and keeps her busy but it is no substitute for building a deep and meaningful relationship with a child.
All is not lost! You are not a bad mum. It's redeemable with a few tweaks.