Hello, it’s quite tricky to do a correct calibration, as many parameters can vary. The procedure you describe seems correct, yet, your images seem to suffer flat overcorrection.
Flats are supposed to be shot with a short exposure, with a lot of light, they don’t need to keep the same ISO/gain or exposure as lights and darks, but their offset value needs to be understood and subtracted. If you’re not sure about the offset level, you might just take bias shots.
Darks must have same parameters as lights: ISO or gain, exposure, binning, temperature (or close enough), offset.