An Update - after some more due diligence and reviewing other forums - this issue appears to be very common. I even performed a restore of a HomePod running 18.5 via my Mac and still no ability to update to the TVOS 26 beta Several forums have indicated that if you are not a member of the Apple Seeds program - that you'll simply need to wait until the public beta... I've also noticed that even on Apple's Beta site - when you click on HomePod Software - the page still referes to 18.x - not 26.

Was able to successfully update 2 Homepod minis to HomeOS 26 public beta build 23J5316g. If your homepods are on an 18.6 beta build restore them back to 18.5 and rejoin the beta