I've had the mini hub and four mini speakers for a while. I had issues with voice matching and couldn't get it to work. There was a link in the Google home app that took me to their website. A community forum is there so I signed in and searched for the issue. Lots of people were having the same issue and a Google community moderator acknowledged that they were aware of the issue and provided us with a temporary workaround that worked great. Just an option to consider if you don't find the answer here.
We did test the mini hub and speakers by talking from one room to another and the conversation was loud and clear. The mini hub is one room away and has a really strong wifi connection four bars all the time. The mini speakers vary with two or three bars depending on how far away they are. I don't know anything about the camera yet but I've been thinking about buying one. There pretty expensive so I'm glad you posted here about the low volume issue. How is the video quality? Can we cast it the big screen?