Deniable encryption mixes more poorly with SSD wear leveling than you believe it does.
The secure element has built-in storage protected against tampering. It doesn't rely on external storage. Having support for the duress PIN/password built into the secure element rate limiting would force an attacker to risk wiping it unless they have a secure element exploit. If they have a secure element exploit then they don't need any attempts for a typical PIN since they could quickly brute force it. That's the planned design of the feature once we have the ability to extend the secure element functionality.
If they can exploit a secure element then only a decent passphrase is secure. GrapheneOS does make using a decent to strong passphrase much more convenient and memorable but it isn't what most people use in practice.
The secure element has built-in storage protected against tampering. It doesn't rely on external storage. Having support for the duress PIN/password built into the secure element rate limiting would force an attacker to risk wiping it unless they have a secure element exploit. If they have a secure element exploit then they don't need any attempts for a typical PIN since they could quickly brute force it. That's the planned design of the feature once we have the ability to extend the secure element functionality.
If they can exploit a secure element then only a decent passphrase is secure. GrapheneOS does make using a decent to strong passphrase much more convenient and memorable but it isn't what most people use in practice.