A wonderfully helpful response, thank you very much. Re-reading the comments on Brent Hall's channel, I see that they are all about the 800/11 and 100-500.
I imagine, though I don't know, that long lenses on APS-C with the 1.6x crop factor would make the AF and IBIS systems work hard and might be subject to more errors. But that isn't something I would need to worry about, as I probably wouldn't use anything longer than 135mm if I got the R7.
At this point I'm reluctant to part with the R6, and budget doesn't allow for owning both. But, boy, that R7 sensor sounds intriguing with its extremely high pixel density, and although I've seen only one portrait that I believe puts its glories on display, it is a winner:
Frau D. | Pascal Volk | Flickr.
Gratefully,
George