I am totally shocked. The new Canon 600mm 4.0 III delivers in any regard and exceeds the already top notch results of the previous version. I combine it e.g. with s Canon 7D Mark II for birding and it is great in IQ, Micro Contrast and coloring.
Purchase a version II used, image quality is comparable, it weighs 2 lbs more but 1/2 the price. Wait for a 600 f/4 or better to come out in the RF line then you may get a version III for less. Canon isn't making EF lenses anymore so at some point people will be switching. .