i feel like this would just be better served by having a phone and a camera. a good large camera will continue to be a good camera for years and years past the time the phone is too old to be useful for modern needs. my almost 20 year old DSLR still outperforms my phone camera, and my phone is quite recent.
my point is that you might as well just have the phone be separate at that point. instead of having to frankenstein them together just have two devices. also, last i checked the linux experience on a small handheld device is not something you’d want to subject yourself to daily. android is much more what you’d want.