I see where you are coming from but I disagree. Using grapheneos will allow much more security and control over the device so OP can fine tune their grandparent’s device to their liking. As for not agreeing with the foss apps I really mean in this situation specifically. Foss apps often run into problems with stability (look at newpipe due to youtube) and ease of use (look at collabora) which will frustrate an older person and with grapheneos at least OP can further lockdown the proprietary apps that their grandparents would use if they do so in the first place. However, since OP stated they just want a phone that’s simple that’s why I would go with the stock grapheneos setup without adding new applications. As for compatiability, unless they are driving or banking which I doubt since they just know tha basics of a smartphone, I doubt this will be a problem since the majority of mainstream apps work. Grapheneos is meant for freedom and securiry over your device not just privacy
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