You shouldn't give a vitamin supplement separately if you are already feeding a fortified pellet. That is because you can actually overdose on vitamins that way. It is really hard to overdose on vitamins in pellets; much easier with a separate vitamin supplement.
Harrison's is backed by decades of research and feeding trials, as are Roudybush and Lafeber. Harrison's is organic, has a zero tolerance for aflatoxins (the main reason to avoid peanuts), and is dye and sugar free.
Roudybush is also dye and sugar free.
TOPS is organic, corn, sugar, and dye free, but it is lacking D3. They claim they have it as alfalfa and seaweed contain small amounts of D3, but everything I have read leads me to believe it is not a sufficient amount. If your bird gets unfiltered sunlight (ie, not through a window) several times a week year round, then your bird may not need a fortified pellet. However for a lot of us, that isn't an option for one reason or another.
There is no perfect pellet.
Pellets for Parrots
Here is my info on D3:
Vitamin D3 for Our Birds
There is nothing inherently wrong with corn and millet. It's an excess of anything that is bad.