graphy00 I would do the install and setup for them as you mention, to try to use Graphene at least in the family, but I'm not in the same country, so after setting up their phones I can't do anything except spend hours with them trying to guide them through things that take 2 minutes, which I do sometimes, but usually they are not willing to do this.
I find that people can be extremely lazy when pushed beyond their comfort zones especially as it relates to technology. Even getting someone to install an app like Signal seems, to them, to be one of the most annoying things ever.
I personally believe that once a user has access to an app store and knows the name of the app they want to download, they should be able to figure out the rest. There are only three issues I can see that GOS may have that could be related the larger idea behind OP. (The camera quality is the same on Stock and GOS Camera, the difference is the software processing as mentioned above, OP may be talking about some AI features I am unaware of but that remains to be seen.)
Some apps are not available through the play store. Netflix is a big example. It works, but guiding a user who lacks independence and technical knowledge/awareness would be difficult. Not GOS's problem, it is Netflix's and Google's.
Some features, on Stock, that require privileged access are not available. I am not aware of the full scope of these but I do know they exist.
It is not immediately clear to the average user upon setting up the device where they can get their apps. Google shows up in the App Store as Google Play services. I can almost guarantee that the average (your granny or uncle etc) user does not know what that is. In fact, the average user (I have had this happen to me when I let someone use the guest profile.) will just see the list of apps, not see the Play Store, and give up!
The GOS team is doing what they can with the first two "issues" but the third one is the only that they can actually address imo. I think they already do plenty of work here but the only suggestions I could think of would be some direction during onboarding that lets the user know where to get the Play Store and possibly makes the App Store icon visually stand out for a moment OR an option during setup, once connected to the network, to just download the play store before seeing the home screen.
If I got anything wrong please feel free to let me know.