The only customisation has been to switch to MATE desktop every time, which she finds more familiar. That might also imply sticking with Xorg? Overall though her experience has been good. But perhaps your grandmother uses a wider variety of apps. For her, it's mostly about videos (she does use the ISO automount feature in MATE and CD ripping), some document editing, a web browser, and video conferencing.

The last support call was about plugging in a projector. Turns out the projector was not turned on, and once it was on, it autodetected just fine. I then mentioned mirrored mode and monitor positioning, and that was about it.

Ubuntu is offering 5 years of LTS maintenance on 22.04, and if in 5 years she needs help with the upgrade, I don't feel that's a huge imposition, but she may well manage it on her own just fine.

I hate to say it, but ... don’t? Go the full managed-workstation route, put an LTS on there, and hope you’ll have a chance to do a version upgrade sometime in the coming years. I usually dislike the idea of using an LTS on a personal machine, but here, when the person using the computer isn’t the one maintaining it, it feels appropriate.

I agree it should be better, but it doesn’t feel like a grandma problem, it feels like a smart-and-willing-but-not-savvy problem. (Not that grandmas can’t be in the latter category, it just doesn’t sound like that’s what you’re describing.)

Which distro was this? The major-version-upgrade flow is very different for different distros.

IMO it would be difficult to make upgrading between major versions much easier than Fedora Silverblue — literally 3 clicks. (If you want to try it out, now is a convenient time to install Silverblue 38, because 39 is due out next week.)