@Levina de Ruijter I am starting to make a plan to compare the R5ii to my R6ii and R7.
I know the best test is to use the same lens for all cameras and all shots, my RF 100-500. However especially birds changing cameras on the lens to try to get the same shot with each would be tough or impossible even on a perching bird. One place I could do that is at the local airport, parked private planes about 100 yards away, I was taking pictures of birds and cars at the local mall one time and someone called the cops and they told me to leave. At the airport I could sit where the spectators and photographers sit, they have benches and don't mind you spending hours there taking shots. There I could switch from camera to camera on one lens.
Another place is the pond where the families feed the birds, I can sit there and get shots with the same lens on different cameras.
Another option is to use each camera with it's own lens, I would do something like the RF100-500, EF100-400Lii, RF 100-400, all shot at 400mm. The downside is the RF100-500 is that the RF100-500 is slightly sharper than the EF100-400 and RF100-400, not that much but on the pixel level it is noticeable, also 3 cameras with lens on each becomes a lot of gear to carry
To me that is the best way as it limits the differences by using the same lens on the same subject. Even better is to take the stationary plane pics with a tripod!
So I will probably carry the RF100-500 along with the R5ii R6ii and R7 and a tripod.