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