Hi nebcool,
We appreciate you reaching out to us. We understand that you are looking for a way to manually assign a primary Home hub.
There is no feature that allows a user to manually assign a primary Home hub.
We strive to bring the best personal computing experience to customers around the world through our innovative hardware, software and Internet offerings.
We welcome your feedback on our products here: Product Feedback - Apple
If you are asking about this feature because you are experiencing an issue, please try the steps under the "If your home hub isn't working as expected" section in the article below.
Set up your HomePod, HomePod mini, Apple TV, or iPad as a home hub - Apple Support
Take care!