I use the R7 routinely with the RF 100-500mm, and don't find the R7 too small. So, the size and ergonomics are personal. The R10 has several problems for me as a back-up for the R5: the different batteries, especially the R10's being smaller; the lack of a protective shutter when changing lenses or putting on extenders; and a very slow read-out time of 40ms for a 24 Mpx sensor, even slower than the 31ms for the 32 Mpx R7, leading to rolling shutter. As a stand-alone small body, the R10 would fine, especially with small IS lenses that don't require changing often. The same could apply even more so to an R8 of the form you suggest.
I have a lot of spare LP-E12 (M) and LP-E17 (M6II) batteries, so that isn't a big issue for me. I hadn't realized that the R10 lacked the protective shutter, I really like that on the R5. I've only had to dry-brush off a single speck in the past 2 years. The RP needed cleaning almost weekly, till I switched to the filter adapter.
As for the size thing, I find that I like to have my pinky completely on the grip (e.g. R5) or comfortably under it (e.g. M6II). The RP and R7 were in the range where it didn't fit completely under it and it started rubbing. The RP has the most excellent EG-E1 mini-grip, that made it very easy to switch between a camera fit for the 100-500 and a much smaller body for the RF50 STM to stuff in a large-ish coat pocket. I'd probably go for the RF16 nowadays.
I searched around for EG-E1 equivalents for the R7 and found one that I could quickly and cheaply get before the rental body arrived. I 3D printed this:
https://www.printables.com/model/312874-canon-eos-r7-grip-manfrotto-200pl-and-screw and it made my pinky much more comfortable:

With one or 2 more iterations and shortening a 1/4-20 screw this would be at the level of a proper EG-E1. But in the end it came down to: Do I really want to spend €1500 on a replacement for the M6II, when it won't actually replace it, but turn into a 3rd body to bring?
I did enjoy using the R7, but at the end of the macro season it tipped into GAS territory. Maybe next spring, after all the rumoured announcements, the R7 does turn out to be the 'best' replacement for the M6II and I'll see if I find it worth buying. The R5 is such a good camera that anything I'd get as a 2nd body pales in comparison
