Okay, here is a picture taken with the EOS R5 that shows the real uniqueness of the camera and the AF in an impressive way. The camera's AF does indeed find an animal that the photographer cannot see and focuses on it. I am absolutely impressed.
Okay joke ends here.
I don't know what happened here, I think the camera was at least as surprised as I was to see 2 rabbits and a fox at once.
That said, there is no better (tangible) way to reach 1200mm at the moment, so in most cases I accepted the softness for additional reach on subjects that wouldn't let me any closer. The additional reach was definitely noticeable, and images like the robin I posted didn't even need to be cropped. Some sharpening on the 1.6x crops made for excellent results.
Most of these are downsized outside of photoshop with a basic resizing program to 3000 px, which probably also doesn't particularly help with the softness.