Edit to my post from yesterday. After several failed attempts, I restarted the computer and tried update as first action. It started partially downloaded (1.6 GB) then finished downloading and I was able to update. Of course, that set all my main Apple apps to update available (pages, GarageBand, etc...) and now they just spin their wheels instead of updating.

I am having this problem on an M1 Mac mini which I started up yesterday. Plenty of hard drive space. Have it hooked to internet via ethernet. Personal computer - not managed by a company admin. Gets almost to the 3.43 GB mark when it fails. This is...frustrating.