To give you an update, I have now returned the RF 600mm F4 and will be only keeping a 100-500mm Zoom with my Canon R5 Mk 2.
The reasons for returning were:
1. It is big, in fact it is one inch longer without the hood and the hood is huge, easily extra two inches on that of a Nikon 600mm F4 TC.
2. This is a real kicker: I have found that in my lightly used copy, it would fairly regularly refuse to move the focus electronically. It happens sometimes directly after switching on the camera or, pretty regularly, when focusing far, then close, and then far again. The latter would not work until I pump the back button many times, change the aperture, go to another preset, it could take half a minute of fiddling to get it to focus. It especially refused to budge if there was no obvious subject in the field of view. Now, before you say it is just this copy, it may be worse than usual, however, the shop people have confirmed that the Canon RF 400 and 600 have dual motors and R5 (even Mark 2) cannot 100% reliably drive them. It is likely that R3 can and most certainly R1 will be able to. This means this is not uncommon and one just has to live with it. Now, I've checked that the big Z Nikons both focus instantly wherever you point them, to me that Canon behaviour felt like a major downgrade.
The reason for not returning would have been excellent photos _when the body+lens worked_--I do think that my RF 600mm was better than Nikon 600mm TC.
I have now replaced this RF 600mm lens with a new Nikon 400mm F2.8 TC Z S.