you got a lot of answers already but a few thoughts:
On Phones:
I wouldn't go before pixel 7
-pixel 6s got the first line of in house google processors (tensor) and they were good but tended to overheat
-pixel 7 really good phone and camera and may be able to get a cheap used one on swappa.com, they are a little slow compared to the modern ones so it depends
-pixel 8 and pixel 9 are both excellent and took big steps forward in performance and security
On Graphene:
-installation of graphene has gotten really easy, basically just connect the phone to a computer and run some software
-by default things are more private but you can run most droid apps, you install them through aurora store and the os does some steps to keep you from giving google your data