You can only install Graphene on a Google phone and there's two big issues with that:
- you are rewarding Google by buying their hardware.
- it doesn't matter how secure the os claims to be if the hardware is compromised and there is nothing you can do to convince me that a pixel doesn't have a backdoor into your data at a hardware level. If I was a conspiracy minded type I might say that Graphene is a Trojan Horse.