I absolutely love i2p and I am suspicious about Graphene being based on Pixels (for now). That said your family is running fully compromised mobile phones endangering not only themselves but the conversations you have with them. For them it's a gigantic leap forward to use Graphene over anything else.

Afaik Postmarket is not a hardened Linux, so different attack vectors may apply.

We need an open hardware phone which is not possible at this point in time for regulations. You could have a secondary device without baseband.