She has a normal (not admin) user account on her Mac. She cannot install applications*. Her Documents folder is synced to iCloud. Everything is backed up with Time Machine, and for good measure I rsync her home directory to my own backup periodically. She uses a locked down Google Worksplace account (this cost more since we pay for one admin account and her restricted account). She can access everything from her iPhone and her iPads.
Surprises: some applications can be downloaded and will install themselves into ~/Applications, these usually are the thousand or so zoom/webex/webmeetinggo variants. Initially we did not lock down her Google Chrome and she kept installing "Extensions that will speed up your browser experience!" which of course were scams, so we locked her out of installing Chrome Extensions (this has made it difficult to legitimately install chrome extensions unfortunately). She has, through what I'm guessing were bad UI questions for someone now in her 80s, wiped out her Documents folder multiple times, syncing the deletions to her iCloud. Minimal actual loss since thanks to my paranoid backup strategies.
Hard: she's been using Macs on and off since the 1990s. Each new recent release of MacOS or iOS has become a bit of a nightmare. While I get the relentless need to upgrade and improve and sell more widgets I really wish there was an LTS strategy we could opt into that was just security or sev 1 style bug fixes.
Really hard: I fell into this because every time she went into an Apple Store or tech shop for support they 1) would ask her what her Apple ID was and then just go off and create another one for her anyway. I found six before I got her to understand it's just her email address. 2) Every tech guy has his own way of doing shit, myself included, and it's utterly baffling to someone who simultaneously has been using computers for decades and yet is very much non technical.
She's in her mid 80s now, I think we're on her last MacBook Air, which I just moved her to in the past year or so. The next upgrade, if necessary, is probably to a Chromebook.