This is not exactly what you asked for, but this is planned:
https://github.com/GrapheneOS/os-issue-tracker/issues/28
Essentially, you would have a primary unlock method, which can be a PIN/Password. You can make that a long password or diceware passphrase to ensure high entropy and eliminating trust in the secure element. The primary method is required after a reboot, as the phone is in BFU state and will only accept the primary unlock method.
The issue above proposes changes to the secondary unlock method, which can be a fingerprint right now. The feature above would add a PIN to that, so instead of having to use your primary unlock method or only a fingerprint as a secondary one, it could be your primary unlock method, or a PIN + Fingerprint combo.
This ensures that again, you can use a high entropy password/passphrase as your primary unlock method which may not be convenient to have to input every time you want to unlock the phone, while bolstering the secondary unlock method to provide another barrier to unlocking the phone on top of just biometrics.