Hamsters will not naturally overeat Fiz and will choose themselves a balanced diet so don't worry about that. At 12 weeks she is very much a young growing animal and far from fully grown in terms of her body bulking out and muscling up. You should be generous with your food and also offer some treats like protein rich food such as plain chicken, dried mealworms, boiled egg, tofu, cooked pasta, porridge etc.
Personally I think that is a little much veg as she will fill up with cucumber which is water basically and not eat her sed mix.
A fingernail sized bit of one type of veg is plenty 3 times a week or so I'd say.
A syrian is not really fully grown and a peak weight and condition till 6months -9 months I would say. Skeleton and actual body size at around 4 months but they do fill out after that.
Let her eat as much as she wants and she will govern her weight correctly
