If you think finding a 500mm F5.6 PF is hard, try finding an R5. Seriously, I know someone who traded in all their Nikon gear for one and has been cameraless for over 2 months.
Again, as for me, I found the greatest upgrade I've made for birds is moving to primes. Focus is quicker and more accurate, OOF areas are much more pleasing, and with PF glass I can handhold all day.