Looking for some advice on how to complete installs. e.g..... silent install of Catalina is performed, and after the reboot you need to login to complete the install (I assume I'm not the only one seeing this) and that takes about an additional 10mins. Is there a way to do the complete install / upgrade without having to login to complete?
Unfortunately there is configuration that has to happen post login. The closest you can get to fully automating it is agree to license and no interaction which does not remove the post login part. Your best bet here is to submit a feature request to Apple to add that functionality to the "startosinstall" binary.
You already have this, but just putting it here for reference.
/Applications/Install macOS Big Sur.app/Contents/Resources/startosinstall’ --agreetolicense --nointeraction --forcequitapps