Having two separate SSIDs and joining them to the 2.4GHz SSID also works, but it’s kind of a pain and you lose certain functionality if they aren’t on the same SSID as your iPhone.
I went through two different WiFi-6 networks, in every possible configuration, with no luck. Cumulatively I must have spent 20-30 hours debugging the networks, and finally happened upon something that works, albeit with caveats. I got two Eero Pro 6 access points, and disabled band steering — it was an experimental setting at the time — and everything worked! I've since re-enabled the setting and immediately have issues so I feel confident saying band steering is the issue.
Since then Eero has updated the setting to include AP switching and it's called "client steering". So, unfortunately, to have my HomePods connect I sacrifice AP handoff and have to toggle wifi if I move my laptop to the other end of the apartment.
There are quite a few folks who have brought this up in an Apple issue, and I have a friend in Tucson — where radio interference isn't an issue — with the same problem.