I’d like to see a comparison between the 100–500 with a 1.4 ET. I’m concerned about the loss of image stabilization at the long end.
For me, I would be more interested in keeping the 100-500 with a 1.4 ET, and looking into an EOS R5 mk II with a better sensor and faster read out, potentially. I regularly shoot in crop mode with the R5 using a shortcut on the camera and get 20 megapixel images. That’s enough to crop and still leave at least five megapixels. The better image stabilization at the Long end also allows me to hand hold shots with lower shutter speeds, in order to, “shoot it dark,“ and bring up the exposure with less noise in post.
I’m also concerned about the weight and balance at the longer end. Add an extender to the 100–500 puts the weight or extra weight back towards the camera body and the balance is fine.