I found what I think is *my* problem anyway, and it’s kinda weird but - it worked.
As always, YMMV
I have a second home that was also set up in HomeKit, all that home had was Hue lights.
I had already removed it from my Home app - it didn’t show in the app itself.
However in the “Home settings” the other house still showed up - even though it was not selectable in the app itself.
So I removed the other house from Home Settings and then I was able to do the upgrade.
Also - I had found this earlier, it didn’t seem to fix the problem but I’ll mention it here - I also had a really old AppleTV showing up on my iCloud, but we don’t use it anymore. It’s literally in a box in the closet. But iCloud showed it as on OS 15.4.1 and associated with my account. I removed it from my account. That didn’t fix the problem - but who knows.
Ultimately it sounds like two things:
The “upgrade” is *really* picky.
The error messages erroneously say HomePod updates needed when maybe it’s not actually the HomePod that needs an update.