I believe I may have this solved for at least my situation or to a point where I can live with it.
Again I have very unreliable DSL service and trying to find a solution to allow me to at least stream from my phone and mirror to a television to keep from using Hotspot Data which my provider caps at 15GB hotspot data before they throttle.
I found an idea from another forum and so far appears to be working for me. I may see different in a week or so but for now this may just work.
What has worked for me so far is this. Setting up my Pixel 2 for Hotspot, creating the Wifi Network. I will call this DeviceA.
Hook up the Chromecast to the television ready to be configured.
Connect DeviceB to configure the Chromecast. In my example it is an old iPhone 6, which I downloaded the Google Home App on.
I used DeviceB to connect Chromecast to the Hotspot's network.
After the Chromecast is connected, I no longer needed DeviceB, I just turned it off or disconnect from the Hotspot.
The problem I kept running into was the requirement for the Chromecast to have Internet Access - I kept seeing the message to connect to the network. Ideally I would not want the Chromecast to do anything except mirror DeviceA directly without having a Wifi network at all. But since that does not appear to be possible at this point... I move on.
What I have found I can do is when I am ready to watch Netflix, Hulu, etc. I can turn on the Hotspot on DeviceA - which gets the Chromecast internet access, then turn on mirroring through the Google Home app on DeviceA - once they are connected and mirroring, I can launch Netflix, Hulu, MoviesAnywhere, Prime Video, etc. and use DeviceA data to be entertained and mirror to the television while using minimal activity on the Hotspot. Which I can live with, looks like it is using about .03GB where I was seeing a lot more being used when streaming from a Roku using the Hotspot.
There are likely better methods, but this may make my household happy. And make me happy by not killing all of my hotspot data in a week of streaming television. Which we can save for work, school, etc. and while not perfect but may allow us to drop the DSL service.