So after getting new iPhone today I un-paired my Series 6 and when I re-paired I set up as new. It appears that Announce Messages is now working after I set the watch up as new and restarted. I made an observation based on previous comments here that when I placed on charger and went back into settings and Siri voices that it was downloading the Siri voice (this was without changing accents or male to female etc). This may explain why some here were successful after it sat overnight. It appears that when setting up a watch for the first time it needs to download the complete voice assets before the Announce Messages feature is available. Which makes sense for the feature to work in standalone when the phone isn’t nearby. It seems that setting up the watch up as new rather than restoring from backup also helps. Anyway, wanted to pass this along.
On a separate note, to use the Unlock Mac with Watch feature, try having your phone nearby and unlocked when you turn this feature on for the first time on your Mac in System Preferences > security and privacy > general. Have a great weekend.