....and this is only Google phones solely because NONE of the alternatives meet the team's stringent security requirements.

The graphene project seems to choose security over freedom in a few cases. They also recommend using the Google Play store over F-droid IIRC.

Not my preference, but they seem so far ahead of other ROMs right now that I use it still.

I do believe people have built and installed it on other devices without too much trouble, but I don't think that'll ever be supported.