Home App No Longer Working on iPhone 7 Plus

I have an iPhone 7 Plus running 15.7.3 and an iPhone XR (iOS 16.3).

Last week I bought and set up a HomePod mini on my iPhone 7 Plus and it worked well via the Home app. I then installed and opened the Home app on my XR and it was set up via the iCloud settings created by the 7 Plus. The HomePod mini and Home app were working well via both phones.

Yesterday I added a second HomePod mini for a stereo pair but this time I added it using the XR and it was working well but I did have to delete and re-add some things in the Home app. Then when I opened the Home app on my 7 Plus there was a problem.

I now get a green screen with an exclamation mark inside a triangle and the text:

This iPhone Can No Longer View This Home

This home requires software that isn't compatible with this iPhone.

If I tap the + to add a home it says it cannot be created. I'm assuming that the changes I made in the Home app on my XR replaced or updated the original (iPhone 7 Plus) Home settings on iCloud to a version now only compatible with iOS 16, so the 7 Plus will not update or work.

I've searched and tried everything I know of: deleted the app on both iPhones, turned off/on Home sync in iCloud, deleted the iCloud data for Home, turned Keychain off and on, restarted both phones, reset and turned off HomePods, plugging one back in, and more, many times, but none of these work. And now I can't even delete the Home app on my 7 Plus. It says I must delete any HomePods in the app, but there aren't any and the app isn't working. I'm using the same iCloud ID on both.

The HomePod minis and Home app work well on the XR, but no longer on the iPhone 7 Plus (even tried on an iPhone 6s Plus with the same problem).

Any ideas? Thanks.