I tried everything and my bettas can't see the food anymore in the dark, but they can see the congregation of movement from the corydoras in the dark. Then they go down to the general area and managed to pick out the food.
I resorted to just feeding everyone at the same time. I drop in enough sinking pellets that my 7 bettas can't eat quickly enough before it hits the bottom (I use pellets a bit smaller than betta pellets and they don't seem to get bigger when wet). My bettas have now learned to wait until it all hits the bottom and then they go after it (every animal seems to have a lazyness ability). Everyone gets fed and the corydoras know where to congregate when I approach the tank.
The shrimp also know to come get their piece and then they run away with it. My upside down catfish have become a new problem because they eat faster than the bettas do (they're freakin vacuums). So I have to feed even more now so everyone gets a share. To keep the balance I now only feed every other day and skip weekends.