I'm curious what about Graphene you think would prevent certain demographics from using it as a daily driver? There are pretty well no downsides to using it compared to a first party ROM aside from not having certain things like Google Assistant baked in. It has automatic updates, you get the play store and services if you want them, it even has android auto. It's also the most rock solid android experience I've had since I switched from iOS in 2012 or so.
Obviously a stereotypical grandparent would need someone who knows computers to do the initial install and setup, but after that it's pretty well just set and forget.
I've been using gimp's 3.x branch since 2016 or so (after getting a hidpi display) and gimp itself since the early 2000s, both for personal stuff and for work. I'm typically editing existing photos and images to clean them up, apply effects, make new clean images from pieces of existing ones, etc, and for my uses it's great. Also, having been using it for so long, I actually really prefer the ux to Photoshop (especially since they added an option to use it in single window mode).
I've seen videos showing some of the features it's missing for certain types of things though, and while there are hacky scripted ways to emulate them, you might find it lacking if you're expecting those particular features.
I'd recommend looking up tutorials on YouTube for things you frequently do and see how much work it is and what the final product looks like. You could up the playback speed to save time since you won't be following along with gimp yourself.