Thanks Mads,
I think I must be doing something inherently wrong in my workflow:
I tried the Flat Calibration Wizard and it took a bit of time to get all the filters to get calibrated.
At my lowest light setting and lowest Gain, I set the Minimum exposure to 0.5 sec and max of 10 sec.
The calibration wizard can save the settings to a Profile or to a Sequence.
In either case, the exposure times are auto-filled and were all lower than 5 seconds.
With the ZWO ASI1600MM cooled, anything less than 5 seconds results in horizontal banding.
Any suggestions how to fix my workflow and the banding would be much appreciated!