I have 3 Chromecasts, 1 Google Nest Max and 1 Google Nest Hub (2nd gen) in my house.
I have the Google Cast integration installed in HA.
I see all the devices in HA.
My problem is that my Google Nest Hub entity has the status “Unavailable” and I can’t change that status.
Therefore I am not able to cast anything to that device.
Casting to the other devices is no problem.
I have done all 3 suggestions, but without result.
I can cast to the Google Nest Hub from all other sources withou any problem.
It is also available in the Google Home app.
I have moved the nest hub to the first floor of my house and now HA sees it and is able to cast dashboard.
I don’t understand it, but the problem is solved.
I had the same problem.
FIx for me was, changing the DNS server on HA host machine to not use the internet provider DNS, but used the local DNS on the modem/router (192.168.0.1)