Thanks Rick and Ray.
Well, after trialling ALL my flats, it seems the most consistently well performing were indeed my 17000 ADU ones, and more so, the ones I shot with the light box VERY dim (in the old standard, this would have been sky flats taken pre-dawn) and 40 seconds (
). If I did my "old standard" of medium brightness and around 3 seconds, it seemed to in fact introduce some shutter shadow issue and noise, even with the shutter pause set in MaxIM.