I don’t think there is a magic bullet – and yes I have completely considered adding parental controls.
I think there’s probably two prongs of attack. Helping them manage their IT and Scam prevention. Scam prevention covers cold calls “from your bank”, random letters in the post, people knocking on the door etc. IT competence is supplementary and confidence here helps prevent the former. e.g. If you’ve installed every toolbar offered to your browser, then a) You shouldn’t be in charge of a browser and b) Are more likely to need the help of MS when they call.
Things I’ve done, in no particular order:
Offered to be their IT support. If in doubt over anything, please call me first. I don’t mind, it’s how I can be helpful and show gratitude.
Added their machines to my Google One Backup (or whatever your backup solution of choice is with an online family plan). I’ve tried leaving them with USB drives to plug in and local backup scheduled, but never seems to work out.
Accept some people shouldn’t own a PC. Chromebook/ipad provide most of what they need and are relatively sheltered.
Push them towards online services for say email. Yes, they might be used to Thunderbird that you initially set them up with – but de-corrupting local storage, missing emails from that time they accidentally used POP, hooking in AV, anti-spam etc etc. Gmail (or your provider of preference) handles that for you (and you can just use thunderbird with that if you insist – and it will grab mails from that ISP account you mysteriously are attached to).
Education. Quite surprisingly my PC-cautious relative (never messes up, but refuses to embrace) decided to take a “Computer Driving License” course. I was slightly disparaging to be honest, but she found it interesting – and started realizing what she could do. e.g. Address book previously a txt file (kept on a USB stick for security, naturally), made the switch to Excel and mail-merged the envelopes for the Christmas letter.