how to change default hub to home pod mini

Go to the AppleTV settings "Airplay and Homekit" and disable it as a Home Hub (changes from "connected" to "disabled". Then Homekit will always use the Homepod Mini.

Note the settings menu has changed in a recent tvOS update. If you are running an older version the menu may be different.

It most likely needs to be started from scratch. Reset the HomePod to factory settings Reset HomePod – Apple Support (AU)

Set up and use HomePod mini and HomePod – Apple Support (AU)