I can't see how that is possible without compromising on their security requirements. The hardware isn't the problem here, that could be overcome, the extremely rapid firmware updating reqirement is. That needs huge resources, hence they are using so far a device from the huge comoany behind the Android ecosystem. Already Motorola, another huge compaby is stretchung that requirement, as tgey are not sitting at the source of that ecosystem.

Again, I am not defending GrapheneOS' choice, just describing their dilemma