Ok so firstly you want to feed your Parakeet pellets. They are whats best diet wise. Seeds should only make up 10% of the diet so you could just feed seeds as treats because they are fatty and unhealthy. Just feed millet occasionaly as a treat also.
Its a struggle for most getting them to eat fruit n veg but i find mine likes sweetner things like red apples and green grapes, pears so i tried sweet vegetables like corn and it worked.
You just have to keep trying different things. Theres lots of recipies and things on the forum. You can make chop and birdie bread ect, keep us updated x