I am a Fedora fan, but I do think that they may have issues with needing to upgrade every year. I am currently holding off on a Fedora 40-41 upgrade on my school laptop until finals are done, just in case something breaks.
I’ve been on Fedora since about 32ish? And I’ve had a couple of times where the upgrade didn’t go smooth.
For grandparents, I’d probably lean towards Debian with Gnome.
Yup! Security updates, but not build updates. You’re good until 23H02 goes out of support, then you’ll have to reinstall 11. At least, that’s what I understand. My 21H02 machines just never get the push for 22,23,24 etc major build updates.