I'm going to come at it from the perspective of neurodivergence (and experience).
IF she is undiagnosed ND, especially as she is very able academically and has incredibly high spatial awareness and excels at visual memory/understanding, it's possible that her life, her thoughts, her everything is in a virtual 3D space around her. Does she wave her arms around when she's thinking and speaking as though she's plucking memories out of the air? If so, she may be subconsciously aware that she has object impermanence, where if she cannot see something, the memories fade and therefore the good things that are part of that object, the games, the being proud and people saying she had done very well can be lost. It could be that whilst she is incredibly good at masking, it's on the basis of her being able to come home to relax mentally and see herself rather than the way everybody expects her to present.
Being absolutely brutal with her would then be taking away the feelings of being good, clever, hardworking or special. Her certificates could go into a folder or foolscap boxfile and artwork could be placed into A3 portfolios and kept safe or in clear, lidded tubs and placed into the loft. They may be the things she is most attached to because they are the things that prove to her she is special and trashing them out of hand would be felt as you saying she isn't special and isn't allowed to remember other people saying she was.
With toys, it depends upon what that toy means to her - it could be the texture or softness that she's holding onto, a feeling of being safe and warm, or a tactile reminder/sensory experience. What are the types of toys that she holds most? Soft ones, organised boxes and sets, a giant tub of LEGO to put her hands through, a battered pink plastic monstrosity that she would play with when she got home from school as you cooked dinner, something that was pleasingly smooth or fitted together with other items, that sort of thing? If there is a most valuable characteristic, you may be able to identify the items that are the least valuable to her.
Telling her that she's stupid or far too old to be attached to something because it's for babies/give it to other children as some posters have done would hurt more in this context - and could trigger a panicked response where everything must be held onto in future.
But introducing a possible hierarchy where she can differentiate between items and feelings can help - for art stuff, we could keep all the packaging and wrappers, but these crayons are broken; that's fine, crayons don't need to be perfect when you use them - how about so you can always find them when you want them, we take the paper off the broken ones and put all of the crayons together into a tub? (also gives her another experience or memory of the texture, sound and smell of going through a tub).
After some things could be transported or transformed into more or less essential, rather than set exactly as they were, you may be able to identify with her other things that can also go.