

A mask ROM is a kind of read-only memory where the desired data is directly etched into a semiconductor during the same photolithographic process that actually makes a circuit out of a planar sheet of silicon. It’s pretty much hard-wired data storage, with an indefinite lifespan.
There’s no easy answer to your questions. It depends on what threats you’re trying to defend against. If your primary concern is adversarial law enforcement with Cellebrite et al., a current iPhone, especially with Lockdown Mode enabled, is certainly the next best thing to Graphene that we have.
Personally, I have access to Private Relay, but never use it. That’s not because I don’t trust it, but because I only ever use VPNs to spoof my GeoIP. And you can’t do that with Private Relay.