This repeated MacOS upgrade fail annoyed me. I saw stuff on the internet about Apple’s servers being overwhelmed. That wasn’t it. It turns out I only had 4G of free disk space left.
I’m never getting only 128G of SSD again. Deleting 8G of Spotify cache, and it worked smoothly, if slowly.
Edit: had a major fail going to 12.4. The system locked, unbootably so. I took it to the Apple store and the very nice man poked it with some electrons and the update completed.