It looks like the verified boot security feature of Graphene effectively prevents rooting the OS. I understand wanting root access, it does provide some nice features, but I don't have any need for it. I don't have any bloatware embedded to remove, and I don't need to mod any system apps, so I haven't looked into it much. I know the dev says it isn't planned because it massively increases attack surface, which I personally agree with, but it would be nice to have the option via a separate version of the OS or something. If you need root access, I would suggest looking into LineageOS. It's similar in privacy to Graphene and last I knew could be rooted. Graphene is very focused on security as well as privacy, and for me is a best of both worlds, but if you want to modify the system for various power-user type features, it might not be for you.

I'm not sure what the GrapheneOS stock camera app does under the hood, but if it's not enough for you, you have the option of installing Google's Pixel Camera app from the Play/Aurora store if you want to compare. I don't imagine it would require Google Play Services to run on devices older than 8 since they don't have the AI integration, but I could be wrong. You can easily deny the app network permissions to ensure that the app isn't sending your photos to Google. As far as the AI features go on newer devices, I could see those requiring Google Services installed to work, but again, they're available through a Graphene mirror, run sandboxed for privacy, and can be denied network permissions. I'm satisfied with how my pictures turn out (7 Pro), but I may try Pixel Camera out just to see what the difference is.