Great!
Personally i have a TicWatch Pro 3 Wear OS operated smartwatch, and my own smartwatch App for Homey, Android en Wear OS. So i speak directly to my house by pressing a button on my watch (and holding it, or pressing it twice er shaking it in a certain way) and i can speak directly to homey. It uses the build in translate modules from Android, and then sends the recieved text to a custom capability of my Smartwatch device.
So i don’t have expirence with Google Home devices and i also will not be getting expirence with it.
I also re-wrote the (speech)text-to-action module on Homey, thats actually where my smartwatch app started, because in the old Homey world, if i said: turn on lights in livingroom; if it didn’t understand livingroom correctly, it would turn on all the lights in the house.
With 6 small kids, that was not ideal in the middle of the night…
After having the option to handle speechs my own way, you can understand my programming view of homey is probebly very different from yours. Where-ever I am (in the world) I can talk to Homey directly and within way less then a second it will respond. I completly use speech with my Homey, and it works so well now a days, that my wive even “requested” that i would put the tablet back on the wall and create a button for her to speech (she doesn’t want a watch on her wrist all day with the kids)., because she became dependend on my. She would come to me for asking to put stuff on the shopping list (wrote functionality to have lists in Homey). The Homey App is almost never opend on the phone: we have speech and (normal) buttons on the walls for lights and curtain’s and the rest is pretty much automated.
This is why i won’t be experementing with Google Home (would work with my kids in the house anyway 
)
About Athom including it in there software, I doubt it; because if i was Athom, I propably would not do that. You say its only a few bucks, but even if that were true, it would stil be a few bucks…
But it aint a few bucks, because it is annual/repetitive. Meaning Athom would eather have to create subscriptions for it, or you are setting you company up to go bankrupt in 20-40 years… Not a very good idea 
.
Also, tracking of the usage per customer would be needed: else someone can (mis)use it for (in example) reading books out aloud. You can now do that (im working on a idea for that) because you can read out 5-6 books a month within your free usages. If Athom would get an paid account, there is no free part (well, a bit, but gonna be reached fast), they pay (extra) for each letter. So if someone writes a Read-Book-Aloud-App, man o man, Athom would financially be destroyed.
So although setting up an account is a bit of work, this way we can all benefit from it (if we want) without someone paying extra money. And for people who can’t for some reason get a Google Cloud account, there are the Non-cloud devices and Say functions in multiple apps. I would say win/win for everyone.
About the volume, not i cannot help you directly (i have no control over the Google Home appliance app and no Google Home to test stuff (and i’m not gonna place a Google Home in my house, sorry), but yeah i am in the process of getting an app (re-)validated by Athom:
H.O.O.P. (Hope) - Homey Object Oriented Programming
Through this app (and its methods and overloads in this particular case) you can easily (i would say) create a method for when the Url is triggered, to save volume in variable, play tts (and wait to finish in condition part) and then reset the volume (if that can be done with Google Home app?).
Or, request to add an argument for volume to the Url actioncard of the Google ChromeCast app.
