Hey folks,
Just an FYI that a Google staffer has assigned issue:

"[Thread] Allow users to change the Preferred Thread Network and select the Thread network for Matter commissioning."

https://issuetracker.google.com/issues/564071942?pli=1

There is a + icon on the top right next to 'Hotlists' which will cast a vote if you'd like to see this feature.

If implemented, this would allow Google devices like the Google Streamer to join an existing Thread network.

As it stands, folks here and on the HA forums are using a series of workarounds with varying success.

Here is one example:

https://www.reddit.com/r/homeassistant/comments/1ql11bl/finally_solved_my_split_brain_thread_network/

I didn't post the issue, just stumbled across it and am posting here for visibility.

Cheers!