my toasters have all been the opposite, once you warm it up with the first batch, the next batch actually gets more toasted than the first one, even with the same setting.
even a kalman filter would be overkill for this. Just buy a toaster that isn't terrible, the calibre of hardware needed for this costs $0.02 for a pack of 10 - thats the level of cheapness needed to make a toaster that bad.