Buying a device that only runs OEN Android is ridiculous for this exact reason.

We need to decouple phone hardware from phone software, as we did with computers.

We do, but I don't see it happening anytime soon. Many banking / government apps and even some games use the Play Integrity API, which AFAIK is starting to require remote attestation for newer devices.

As it's usually not viable to opt-out of those, the solution seems to be having a separate device.