I agree to disagree it’s really not that different from vanilla especially if he does go the playstore route, so a person familiar with android can naviagate grapheneos. The only problem would be if they break their device and the thumbprint scanner would have to be recalibrated. Personally I would just keep it as the stock grapheneos setup with accessibility features turned to max. That way its a dumb smartphone
Edit: forgot to mention regardless of os add some kind of network firewall or DNS solution to block malicious/phishing domains. The elderly are often victims of scams
The foss apps I dont agree with but a stock grapheneos setup is as basic as it gets so even a non tech person can use it. OP can even set it up for them to auto update whatever apps they use through the playstore so the grandparents dont really have to do anything
You can basically make a dumb smartphone with grapheneos so a pixel would be good for them. If you dont need a dumb phone set it up with the playstore and whatever apps they use and boom good to go
Edit: I would also setup accessibility features. Forgot to mention regardless of os add some kind of network firewall or DNS solution to block malicious/phishing domains. The elderly are often victims of scams