I reflashed one of my dongles to the Thread firmware, and fired it up with the OpenThread Border Router app in HA.
Had it join my existing Thread network by sharing the network credentials with HA from my iPhone, so that it would join the same Thread network as on my Apple TV/HomePods. All devices now work as before.
Had it join my existing Thread network by sharing the network credentials with HA from my iPhone, so that it would join the same Thread network as on my Apple TV/HomePods. All devices now work as before.