The R5 only has oversampled 4k at 30fps, or if you enable the 1.6x crop mode, you can get 60fps.
On the R6II 4k60 is oversampled in fullframe mode.
So if you want FF 4k60, the R6II will give you better quality footage and it also doesn’t limit you to 30 minutes per clip.
I’m judging this from an R5 and R8, the R8 has the same sensor and digic as the R6II.