I'm running a Pixel 3a, also on 20.04 Stable, OTA 9, which shows "up to date", but also shows a "Last update" of 6/23/23, which feels too old to be the most up to date stable version.
I also have the same spinning wheel when I try to switch channels, but am having other issues such as being unable to navigate to any websites in DuckDuckGo due to an invalid date cert error
I thought I read in another thread that the only way to update past this was through ADB - I tried downloading the current ISO and flashing it through the Windows UBports Installer and it doesn't look like anything was changed (same version, update dates, etc)