Just to update my post. It took an hour, but the three machines did finally finish the update.Tossing in another report of installer hangs. I have 3 machines I just set up today (all older MacMinis). Nuke and pave installs, fresh 15.0 then immediate 15.1 update, all are hung with "about a minute remaining". Been like this for over an hour on all three.
One of them is set as a software cache server so I'll hold off on any more until that one completes. Hopefully the rest will go quicker pulling from the cache since Apples servers seem to be having issues.