I have a 5D3, 5D4 and 1Dx version 1
My experience is that the 5D4 and 1Dx have very similar low light performance in terms of IQ but the focusing system is better in the 1Dx
Both are somewhat better than the 5D3 although it is very good and you might not be too impressed with the difference
Have you looked at this? I know ratings and lab measurements don't mean everything but you might find it interesting.
https://www.dxomark.com/Cameras/Comp..._1106_1071_795