Oldie but a goodie
When I first got my R5MKll I didn’t have any RF Glass (to date I still only have the one lens) but i had a heap of EF Glass and an adapter.
One such piece of glass was the formidable Canon EF 200mm f/1.8 Prime. 200mm isn’t that great for birds, especially birds of prey but coupled with a 2x TC I get 400mm at about f/4.0 and that is much better. The Autofocus is a tad slower than the RF and of course a piece of glass like this weighs a tonne! But the image quality is just fantastic for a lens of this age.
New Zealand Swamp Harrier, shot on the Canon EF 200 mm f/1.8 with a 2x TC. R5MKll
So as I drift more and more back into Film/Video my EF glass actually comes in rather handy as most of the Video Cameras have and EF mount or the L mount which plays nicely with EF glass too, so I think for now I will stick with the EF stuff.