Hi Sofie-Ann
I have a similar problem with my elder child, who by his 4th birthday had 2 extractions, 2 crowns and 3 fillings. His sister, 19 months younger, has no such issues.
I have looked into everything, and tried most that made sense to me. It sounds like you are very aware of the dangers of sweet stuff, and her general health. Since the treatment the condition of his teeth has stabilised. It was suggested to me by a very wise paediatric acupuntcurist (who also has a daughter with a similar problem) that if children have a growth spurt or are especially vigorous, the body may well borrow calcium from the teeth to use elsewhere to support that growth. Caries is a problem with young people in particular, rarely continuing into their 30s - and that's not just because we get older & wiser with our oral health.
I don't know if that helps. It's another one of those things you feel guilty and helpless about as a parent. I was treated by one specialist dentist as if I had been abusing my child, so please know you are not alone!
Good luck.
Karen