This is a familiar battle that many people have had. The NHS seems to have a blind spot about reactive hypoglycaemia.
But (sorry about this) I'm going to have to shoot straight from the hip here.
STOP GIVING HER SWEETS
Children don't need sugar at any time. They need nutrition, and there's none of that in sugar, and there's precious little nutrition in the kind of pasteurised processed juice she is probably drinking.
If you don't give her sweet foods, her tastes will adjust rapidly and she'll soon be happy eating less sweet things.
All my life, I have known that the worst thing I can do is eat sugar, bread, pasta, potato. Within 2-4 hours I would feel dreadful. That meant no toast, cereal, biscuits, cake, juice or similar for breakfast. I can't even have a glass of milk because it has milk-sugars in it. Instead I would eat eggs, bacon, cheese, leftovers from dinner... And I would feel good all morning. In fact, I would feel good until I next ate carbs.
If your daughter really does have reactive hypoglycaemia then you can control it. You can make her feel well, and stay feeling well. All you have to do is give her nutritious, non carbohydrate foods. But giving her milk, juice and sweets... Well that would make me feel really bad. So I can imagine how she felt.