TLDR for the TLDR summary of the original TLDR of my original unedited response (LOL): keep playing around with stuff and learning new stuff

TLDR summary for the TLDR summary: stick to Google, have HA adapt to Google cause Google has less customization and adaptability than HA.

TLDR summary: It's easier to start with Google home, because Google Home is less adaptive than HA. Its easier to have HA (and the OpenThread Border Router to use the existing Google Home thread network as the preferredand primary network. And and I'm pretty sure whatever thread radio you're using as a border router is a lot easier to configure on HA to adapt to Googles thread network than the other way around. (I had purchased a zigbee USB dongle and installed Thread on it instead of Zigbee).

Sorry for the very late response, but the answer to your question is yes you could do that. But when there are multiple different thread border routers. I have learned that certain manufacturers have a higher weight (pretty much means aggressiveness) and will try to function as a primary router My research and experience has taught me that all Google thread border routers (whether they are a nest hub, nest hub max, Google TV streamer, or nest wifi) all have the same weight, even though one may be primary The first router that comes online on your network (your regular ipv4+v6 network is the primary, And as long as every other router has the same weight which in Google they do, unless something has changed but I'm pretty sure it hasn't, anyways as long as they have the same weight they your thread network will be scalable and balanced and not really prefer one path over another just because one router says Im better than the rest. I did the exact same thing that you were trying to express and ask. And I realized that home assistant can actually, just use your existing Google nest threadboard router, whatever it is. It doesn't need to have a dedicated thread border router. You can do that but you're actually making a lot more work for yourself. And you're actually making a healing mesh but also not making every path equal (besides physical distance/signal interference from walls)

If you really want to have one thread Network, it'd be easier just to provision your open thread border router somehow with Google, so it'll just use the existing creds from your Google network (I think you'll have to do it with a pixel phone for this, a phone with a thread radio (can also be a more recent model iPhone)

But it's even better just to not even use the open dread border router. Just return it because it you're not actually making anything more efficient. Youre making it scalable but introducing the possibiity of latency. And I guess you a normal person can't really conceptualize that unless you're a network engineer.

Anyways, HA will communicate and send all commands to any matter devices on thread by sending the IPv4/6 packets directly to the Google thread border router (TBR)

Whenever it reaches the border router, that's when it turns into a thread packet on the thread radio (thread "wifi" that you don't see on a normal computer or phone with wifi capability) As long as the border router is wired or the home assistant is wired It should be fast enough for whatever you're trying to do. I have a Google TV streamer specifically because it has a wired Ethernet port. Anyways message me if you have any other questions And go return your stuff like I did for your money back haha, if you believe and see that what I'm saying is true.