I’m sorry to hear about the problems you are having with your budgies. It may not be anything you are doing. I don’t know if you have English or American budgies, but both have been very inbred in captivity. They can have a lot of genetic issues and be predisposed to certain health problems. One thing to consider is are they getting the most out of their Nutri-Berries? Do they eat most of them or are they wasting a lot? If you offer too many Nutri-Berries, they will tend to pick out their favorite bits and discard the rest, which can mean they are not getting the full nutritional benefit. In this case, reduce the number of Nutri-Berries you give them at one time and wait til those are mostly eaten before you offer more.
Have you ever offered dark, leafy greens? Try giving them an intact large leaf of any dark green lettuce such as romaine – no iceberg lettuce. Try placing this on the cage bottom and maybe crumble some Nutri-Berry over it. Budgies are ground feeders in the wild so sometimes placing food on the cage floor will encourage them to try it out. Budgies also tend to like the broccoli florets.
I hope these suggestions will help. You are taking great care of them, but if a bird is not genetically strong, it will still have issues.