Well first off, there is nothing wrong with preferring CCD. CCD cameras have not been invalidated by CMOS, and they still have their advantages.
I would be curious to see some of your data, though. Darks, flats, biases/flat darks and lights. I'd like to process it a bit myself and see what I can do. CMOS is different than CCD in key ways, so it does require some differences in how you do things. For one, dithering is often more important with CMOS, and you have not mentioned whether you dither or not or how often. Additionally, dark frames MUST be well-matched, in that they match the exposure time, temp, gain and offset of the light frames exactly, and cannot be scaled.
Now, there are a couple key differences between the ASI1600 an Atik460 that should be noted here. For one, you used exposures significantly longer. You used 300 an 600 second subs vs. 120 and 240 second. So you have 2.5x the signal per sub with the 460. On top of that, the 460 pixels are larger, so you actually have more than 2.5x the signal, you have nearly 3.6x the signal. So if you are comparing 460ex subs with 1600 subs, of course the 460ex subs are going to look better. They have significantly more signal...tough to beat that.
Now, this is not to say that there may indeed be something wrong with the ASI1600, or how the data has been handled. If there was not enough dithering, or if the darks are not well matched, etc. then the ASI1600 data could indeed end up looking worse in the end. However I would be curious to know what 300 second long L and 600 second long RGB subs from the ASI1600 look like at gain 0. The ASI1600 has a 20ke- FWC, while the 460ex has an 18ke- FWC. So you should be able to use the same longer exposures. In fact, with the smaller pixels, you might even be able to use exposures longer than these, perhaps 450s and 900s. Just as a test case, to see if you can use the ASI1600 in a manner that would give you the results you are looking for. It may indeed just be how you are using it, so changing something up may be worth the time to try and experiment before you swap out cameras.
I also want to stress the importance of dithering. If you feel you are not getting clean results, and you ARE using well matched master dark frames that are not scaled, then you should try to dither every frame. With longer exposures at Gain 0, you should be able to dither every frame without much overhead cost. Dithering aggressively every frame, and dithering randomly in both axes, should help smooth out any remnant FPN.
Finally, if you still cannot get the results you are looking for with longer subs at a lower gain and with well-matched darks and dithering, then it sounds like you might want to try another camera. The ASI1600 uses the Panasonic M sensor. While this sensor, IMO, works really well at high gain (i.e. gain 200), at lower gains the quantization error bites you and it has more FPN issues. If your goal is to do LRGB imaging, the ASI1600 may not be the best camera for the job, and if resolution and details are not your top concern then an Atik 383L may indeed be a better option. The IMX183 sensor has different characteristics than the Panasonic M. This sensor, also a CMOS, works much better at lower gains and not quite as well at higher gains. It may be that the IMX183 would work for you. It depends on the size of the field you need, as the IMX183 is more around the size of the Atik460 than the Atik383. The main difference between the 460 and 183 would be the pixel size, with the 183 having pixels that are much smaller (almost 1/4 the area). This might be a bonus for resolution, but you might suffer on SNR.