There's a relatively simple and much more open and secure solution to this: Make physical EU ID cards the attestation source, and require users to tap them against their phone for critical operations (high-value signatures, login on a new device or after repeated authentication failures etc).

That would solve the open hardware/OS "problem" on the device entirely, as there's no trusted hardware or OS signature required anymore. You could argue that this adds the possibility of a MITM attack on the phone (since you don't know what you sign anymore or who you are providing with your PIN, as the card has no display and no PIN pad), but I wonder if mitigating this is worth all the lock-in concerns that phone attestation goes hand in hand with.

As it is, all EU ID cards already have mandatory strong cryptographic authentication, but in a form that's usable only for in-person ID checks (under the corresponding ICAO biometric identity document standards), not for remote ID attestation. This is frustratingly close, but not what's needed.