Preface: I have an R5, R6 & R7. I have the 100-400mm RF, 100-500mm RF, 800mm f11, and both TC's. I came to Canon mirrorless after a decade of shooting Nikon DSLR's. 95% of what I shoot would be considered wildlife.
Are you a casual photographer or a serious hobbiest that wants to shoot a lot?
What's your budget?
If you put yourself in the latter part of the first question then I don't care what your answer to the second one is - buy a 100-500mm (or if you have time wait and see what the new 200-500mm comes in at and see if folks are selling the 100-500's). The 100-400mm is an amazing lens for the price, but it's not nearly as fast focusing, so if you're shooting moving things or birds in flight it will work but you'll have more misses or delays on your first sharp image. This lens alone is what made me sell off all my Nikon gear. But, if you are looking to do this casually then the 100-400mm will work just fine, particularly if you're shooting stationary critters.
Bodies?
Avoid the R6. Yes, it will work fine, particularly the Mkii. But wildlife is all about pixels per critter, and this is where it falls short. If you're going to be cropping you want more pixels.
I started shooting with the R5 and then put it down to give the R7 a good test. After 3 months I switched back to the R5 and use the R7 only in certain situations where reach is important and I don't need to shoot long bursts. Why?
- R7 buffer is woefully small for wildlife. You can get a couple seconds in compressed Raw, but if that's not enough you miss the shot.
- R7 focus system is superior, but processor isn't.
- It essentially has the R3 focus system which is terrific and allows Eye Detect in all focus modes.
- I am amazed at how quickly the R7 grabs the eye of the subject - so much faster than the R5.
- I am disappointed at how much the focus wavers - I've sent multiple stacks to Canon where the focus point is on the eye and the camera wobbles between good focus and focusing on the background. It just doesn't hold focus well compared to the R5.
- On the R5 I can use back button focus to approximate the eye detect of the R7 in all available modes.
- I tend not to shoot in electronic shutter unless silence is demanded and while both cameras suffer from rolling shutter the R7 is far worse.
While the 1.6x added reach on the R7 is great the above makes it impractical as a main camera. But with the added reach and extra MPs I use it when I need reach beyond the 100-500mm but don't want to use a TC. On the R7 the 100-500mm is effectively a 160-800mm lens. If you consider the cost of a 1.4x TC and the 800mm f11, buying an R7 instead and swapping the 100-500mm gives you everything you were looking for with no loss of aperture. That's where mine gets used.
Since I mentioned TC's, the 1.4x works great on both the 100-400mm and 100-500mm (though only from 300-500mm on the latter which I hate - hence my use of the R7 instead). Don't bother with the 2x for wildlife. Long range paparazzi work? Sure.
Now, depending on your timeline and want for a great rig, I suspect the R5 Mkii will have the R3 focus system and hopefully a sensor solution that all but eliminates rolling shutter. So if you can wait for the ultimate rig then you may not want to jump on the R5 until you hear what the next one is like. You could start with the R7 & 100-500mm, because good glass is forever if you don't drop it, and then decide on the R5ii when it's announced, knowing that all of a sudden that lens is going to look a lot shorter to you.
As always, my advice is worth the price of admission.