What are you using at the moment and where is it failing?
I used to have the R5 with a 600mm f4ii and it was far better than and 1d series for BIF but I did still find the eye detection a little too jumpy.
Not used the Nikon but I have heard excellent things about its AF. Again, I once had the 600mm f4e and that was superb.
Best of the bunch though currently is Sony. Problem is, the lenses are expensive and rare on the 2nd hand market. The AF tracking is significantly better than Canon and the zebra function is brilliant. Going the 400mm f2.8 plus 1.4x converter may lessen the blow a little
There are no bad options but it does go back to what is your current gear letting you down with?
Mike