You can only install Graphene on a Google phone and there’s two big issues with that:

1. you are rewarding Google by buying their hardware.
2. 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.