Good comparison! I need to also ask your opinion about OVF vs EVF. Which do you like better?
I own the 1DXIII also. I agree with your points about the dual CF Express cards, unlimited buffer, and the crazy good battery life.
I also own the R5, and I grab it a lot when I'm taking shots around home, especially if they involve people. And it's great for landscapes too of course. But I love love (love) my 1DXIII for wildlife. The R5 eye-detect AF doesn't work all that well on animals other than birds (at least, based on my experiences photographing elk). And with the track-pad style AF-ON button on the 1DXIII, I can quickly whip the focus point around and always get the dot where I want it. So not having the eye-detect on the 1DXIII is fine.
I haven't used the R3, but it has the same track-pad AF button so it ought to be a killer camera. The only thing it doesn't have is an optical viewfinder. I sure like looking through an OVF, but I have to say that the EVF is pretty darn good.
I'm already making plans to go to Mexico in 2024 for the solar eclipse. To view and photograph it, I will use my 1DXIII. The OVF will be a superb advantage, because I get to watch it in real life (I have a solar filter for the partial phase, and no filter is needed for totality). There's no substitute for seeing a total eclipse through real glass. If I were using a camera with an EVF, then I would need to watch separately with binoculars.