I solved it!!
The issue was that there were a few devices that were turned off and not considered in use. I had these setup way back, but my wife's Google Home had never seen these. They included a Chromecast that was turned off, a Bluetooth speaker that was offline and an old device not in use anymore.
So her initial setup for Google home/assistant and Voice match actually looked like it worked, with no errors, but after that, when she went back to her Google assistant settings, there was a notification to "complete setup" for these 3 offline devices, which I thought didn't matter since we don't really control them though Google Home/Assistant.
The speaker that we were having issues with from my original post was not included in this notification, but I guess its completion was being blocked by these 3 incomplete devices. When I removed all of those, and clicked "complete setup" on the notification(even thought the 3 devices no longer existed) her Voice match was added to problematic speaker.
What threw me was that the speaker she was having issues with wasn't listed as "not completed", but apparently all other devices must be online/working for Voice match to fully complete setup.
