A while ago I made the decision I'm going all out R and RF. I'm not sure if I or we will ever know how much difference it will make to have a dedicated RF lens that takes full advantage of the 3 extra pins for high speed transmission. That is one reason why I'm doing this. I have never experienced what it is like to shoot with a 1DX series camera or a Sony A9 II, etc but I figure the R5 is will be pretty close. Today I was going to decide if I stay with Canon or get a Sony birding rig.
Heck the R AF is/was better than my 7D2 and 5D4 with the 2X. Both those bodies were not great at acquiring initial AF in Servo while following a moving object. It also made me question the tracking accuracy after initial AF was acquired (when it felt like doing so). The R focuses pretty quickly with that combo but it lacks the horsepower, etc.
The 100-400 II has been a very good lens for me. As TeamSpeed said the R will AF with it and the 2X and can produce nice work. No bragging about the keeper rate but I always came home with something. I used it quite a bit in Portugal last spring.
I have wanted the R5 and the 100-500 for a while now. After the presentations I was also curious about the 800 DO. As you all know this endeavour is pretty expensive. I sold my 400 DO but I had to if I wanted to move on. I'll miss it but it was never the lens I really wanted.
I called my local store and said sign me up for the R6, 100-500, 800 DO and the 1.4 TC. He said what? All you have talked about was the R5 After a few hours I came to my senses and called him back and said you are right. Switch to the R5 and drop the 800. I figured to keep the costs down and avoid cropping I'd get the 800 to compensate for the R6's 20 megapixels. During those two hours I thought I'd miss the cropping ability of the R5 which I really like to do and I might regret it.
I decided the 100-500 has a nice range. Along with the 1.4 puts me at 700mm and with 45 megapixels will give me the crop-ability to make up for the rest. I'll have a look at the 800 DO down the road once the ouch time of this change settles. I have more gear I will need to sell. Both my 1.4 III and 2X III went today.
I have my R and RF 24-105 and I really like it. Perfect at 30 megapixels for street photography, etc. I won't need burst shooting, etc so I don't see the need to upgrade for a long time. All my gear will be in the Canon camp so I can interchange lenses, etc. I'm glad I got the R and waited it out.