I have looked a couple of times at the image and I get the impression that some of the plumage is not in focus or at least is not showing as much detail as I might have expected. As far as I know the Mk111 converted is entirely compatible with the 500mm lens but full benefit of this TC is with the later series lenses, I wonder if the Mk11 TC would be better suited? Another consideration is the high magnification and aperture of 5.6 would provide a relatively small DOF.
I don't mind the slight "softness" here, which almost looks to me more like slight JPEG artifacts or a little noise reduction, or both. But for me it works as a plus for this image -- the very slight "simplification" effect (maybe it's just flat lighting) and the crop suits the regal pose so beautifully, I can't complain about anything!!
Lovely portrait in a super composition. Yellows do look strong. Could you please share much of a crop you applied here? This image is looking slightly degraded to me, and I have a feeling it's from a large crop. I am not seeing the kind of sharpness and details in the head I would normally expect.