The original resolution of this camera was 11Mpx - 4144x2822 pixels (bin2). ZWO later “unlocked” the bin1 mode: 44Mpx - 8288x5644 pixels. It might be appealing, but you have to bear in mind that in bin1, the pixel size is 2,31x2,31 microns so the light gathering capability decreases dramatically. Just look at the charts in the manual and you’ll realize that the bin1 mode is much less effective. However, you always have it there in case you might need want to use it (perhaps with scopes FL<300mm).
As stated by @vlaiv, good luminance data is paramount. Add as much integration time as you can (you’ll never know what night is going to be better) and finally add some RGB to color the luminance layer. I personally take between 0,5-1,0 hours per color channel.
I use gain 120, offset 30 and -20ºC. YMMV though.
I haven’t experienced any issues with my flats. Following some advice I read somewhere, I use exposure times over 1s: from 2s for luminance to 30s for SII.