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Are you imaging with lights on that might impact the scope? Particularly via any sort of light leaks in the image train? A more brightly illuminated center is not unusual and should be fixed with flats, but the hot center and cross shape looks a bit unusual to me. Flats would still be the first place I would start though.
And yes to dark frames, every time. Low dark current is not no dark current and there will still be some hot or cold pixels which darks will help you calibrate out. I invest a day or two every few months to shoot a new set of darks. I usually take 100 of each exposure time I use, with the senor cooled to my normal temperature and generate master darks for each exposure time that I use and I then use master darks for the following couple of months, archiving the old ones to go with the data shot while they were the current set.
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