Nope and yes, dang 😊
However, I have solved the problem for my system 😊
So, chatting on reddit with somebody with the same problem. He had decided that it was something to do with background apps loading at login which I think is correct because the problem doesn't happen when booting to Safe Mode.
He listed his background apps loading at login - the only ones we had in common were Zoom, CCC and Steam. So, I turned these off as login items - problem immediately gone. Quickly discovered it was the Steam background login app 😊 Left this off which doesn't stop Steam running so not needed as far as I'm concerned.
Interestingly, when rebooting, this had also "deleted" the Wallpaper and Screen Saver settings. I was, however, able to reset these to the previous settings and the "Installer Progress" issue didn't return, although, restarting remained slow - perhaps this will improve over time?
I note that some have found changing/deleting Wallpaper/Screen Saver setting has cured the problem?
Seems to me there is a very low level "bug" in Sonoma, maybe related to Wallpaper/Screen Saver which have both changed in Sonoma, and which interacts with a range of other apps so the problem varies depending on indivudual system setups?