Probably a good strategy overall (especially the part about the winning lottery ticket!). The new 100-500 with 1.4 TC will have a wide-open aperture of f/10 at full zoom, so you're definitely going to need a lot of light to get BIFs at 1/2000 sec without hitting crazy high ISO. So that's the only downside I can see. Time will tell how good the R5 is for noise, but at 45 MP it is inherently limited by the laws of physics so it's going to be noisier than the 1DX no matter what. The only question is how much noisier. Then again, it should be better than the 5DS-R, so given that you have good experience with that already, you probably already have a really good sense of how the R5 will perform.
I think the only thing missing on the R5 which I would have liked to see is the same AF-ON button with trackpad controller, that is on the 1DX III. I find that such a fantastic feature, and it is soooo much better to use than the joystick controller for moving the focus point around. I love how you can swing the point around while pressing the button (I'm a back-button focuser). If the R5 had that I would almost be tempted to get it for my second body. I'm guessing that you should be able to move the focus point by dragging your thumb over the LCD screen (like you can with the Canon R), but that isn't anywhere near as nice as the AF-ON trackpad. A lot of the time you can use auto focus point selection, so moving the point around manually is unnecessary. But when you're shooting bears and other large wildlife, you typically want to go with a single point and then put that point right on the eyeball. So being able to move it around quickly is a real bonus.