I have slowly been moving them, one by one, over to Fedora. I still get support calls, but not related to the OS, more like, "how do I add an attachment to this email, I forgot."
"How do I share my screen?"
"Well grandma, the Google Chrome Flatpak you're using defaults to Xorg so it runs in XWayland, which means it can't see the desktop because the rest of your shell is native in Wayland. If you go to chrome://flags and set prefered-ozone-platform to wayland it might work, but some people in this reddit thread also mention you need to install xdg-desktop-portal-gnome so it can call the screensharing API in GNOME. This also might not work in the Flatpak at all and maybe just the RPM."
Remote support is a breeze now, and we save the usual 30 minutes of guiding an update of TeamViewer over the phone (because of an old and incompatible version) etc.
Flatpak has made it so easy to install most common apps, just press the Super key, search for the app, click on it in the list and "Install". Usually takes less than 10-30 seconds from start 'till the app is installed and running. Very grandma-proof, actually!
EDIT: But I will say that the road to where GNOME / Fedora is today has been a bumpy ride, like a car that is under maintenance while driving :) It's the FOSS / Linux way. In the end, the result is actually quite amazing and user friendly.
I had my ageing parents set up with a Linux desktop for some years in the past. I just -pre-configured everything they'd likely have needed. Browser, email, instant messaging (as it was at the time), a photos app, possibly something else I'm forgetting.
It's the somewhat tech-savvy (or at least somewhat tech-independent) for whom such a transition might be the most trouble.
(And, of course, those people who use specialized applications or hardware for which support may not be available, or who use workflows that require going somewhat below the "appliance" level and which could require significant relearning.)
GNU/Linux actually might've been best because it's harder to do anything in it. Chromebook wasn't an option cause the screens are too small, and idk if Chromeboxes were viable back then (~2015). She used to use Windows, which was an unmitigated disaster with viruses.
As for other issues with basic use, my mother is even older now, and even things that used to be pre-arranged and familiar have become more difficult. But I don't think the OS matters much for that any more, on a grand scale. Touchscreens might help to an extent (and also avoid upside-down mouse issues), and obviously things would need to be set up so that there aren't any unnecessary hurdles. But beyond that it's learned routines rather than any kind of a generalized understanding anyway. She does know how to usually get rid of things by clicking 'x' though.
Did you find something that worked out for your grandma in the end?
(edit: You edited your reply before I finished mine, so I replied to the earlier version. I see the problem with accidentally triggering things. I guess it might be the fairly cautious nature and the limited use patterns that might have saved my parents from them.)
Nothing worked. In hindsight, maybe the Mac would've been ok-ish if I'd used Chrome Remote Desktop to help whenever she had issues, which had fewer pitfalls than the other screen sharing options I tried. Last one I remember was Teamviewer, which broke cause her friend "fixed her computer by removing junk" like that, but it also had other issues before.
Screen sharing for support is probably a necessary problem I didn't think of much since I happen to live close by. It'd absolutely need to be something that requires minimal effort from them.
Sibling commenters have it right - if a computer is an appliance to them, then going for the simplest, least likely to accidentally mess up option is the best one.