I concur with everything said here,as I shot my combo with TC,in less than ideal light this past week-end,and missed a lot of shots,as the lens hunted quite a bit!
Never had it that bad with my canon 400mm D.O,or 600mmf/4,even with x2T.C.,on a 1D body.
Definitely keeping my 600 for warblers,and since I cant sell my DO,(cant even get half of what I paid for),will wait and see what the R5 will be like.If it,s good,and in crop mode with x1.4tc,there,s a field of view of 900mm!That would work for me,or wait till a mirrorless crop body comes out!
I can acknowledge the plunging value of used Canon big whites. I did try to sell the 400DOII shortly after getting the Nikon combo, but as you say, not possible to get more than half of the new price. And sort of glad I still have this great lens, but am seriously conflicted about what body does it justice.
The R5 may be very nice, but will cost another 3500,-4000,-
We'll see. For now, I'll continue to use it with the 7DII when the light is right. I feel free to move up to a serious big lens though (Nikon or Sony), now that the 500PF has given me the ideal easy to carry along solution.
Unless Nikon makes a 600PF, I may well save up, trade all other stuff apart from 500PF in at my camera store, get a 600/4, and enjoy the remainder of my active shooting life!