1st of all Thanks for this project and I hope I will be able to satisfy your need for this project.
So for this project you wanted going for a minimalist interior style which is what you intended doing to create a space for a 12 year old girl. With this in mind I decided to limit the amount of furnitures I put in the space so as not to make it crowded. One way I did that is finding a Single/twin bed for the girl that is fancy and modern that will serve as a bed for her to enjoy. This bed is called SLÄKT. It doesnt only serve as a bed only but as a storage unit for some of her cloths since you said you will have her main storage outside the room. With this bed it reduces the stress of adding another furniture which is the wardrobe there by giving her more space in the room. I also added a simple table and chair beside her bed for a simple reading area. I added 3 wall shelves which in directly above the study table where she will be able to access her books with ease. With this, the room will have more space that she needs and also fulfill her required needs. Thanks look up the funiture lists for the specific furnitures.