It's likely that it won't be possible to install GrapheneOS on any Android phones in the near future. It's becoming quite uncommon for bootloaders to be unlockable. Graphene's backup plan is to try to build their own phones. But that has enormous costs and has a high chance of failing, at which point we're all screwed. The tendency of phones to being totally locked down requires a legal solution. There is not a good technical solution. At minimum we need laws passed to force manufacturers to ship phones with unlockable bootloaders.