Joules - I'm aware of the budget FF announcement, but I feel that's a little too much in the other direction price, and likely feature-wise. As for the alternative upgrade options you suggested, I feel like they all have their limitations, except maybe the 90D... Alongside the things you mention, the 5d4 has a gimped rear screen (no touch, no swivel), the R reviews I'm reading say that it detaches from the SLR user experience when it comes to controls.
Czardoom - I'll miss the extended reach for now, especially as I'll only have the 24-105 to start out with, but I do like night, and have been very limited by not being able to get many keepers above ISO 400. I also like to shoot sunsets, so the extra shadow detail would be useful.
It try not to crop too much, as I like to try and get the composition right in camera. However, cropping would be useful when shooting moving subjects.
I'm not considering the M6 due to it's size and tiny grip. I actually find my Rebel a little small for long shooting sessions, as my pinky hangs off the bottom, and ends up cramping because I'm squeezing my hand.
How do you find the EOS R user experience?