[Darryl Ramm]

I don't know what the problem was so have no idea what to guess the logs would say. But occasionally the logs give a clue what is happening, you'll know it when you see it.

The standard macOS installers can either do an in-situ upgrade (how most people do it) it will default to showing the current boot drive or you click "show all drives" and pick another volume. If that volume has an existing install on it it will do an in situ upgrade of that as well. And if it's an empty or new volume you'll get a full clean install. How I normally do stuff is to have my working bootable volume on the internal MBP SSD, and I'll add a new APFS volume and run the installer and select that volume to do a new macOS install to.

Then it's reinstall apps and moving files across from the old boot volume, but importantly everything is left on the old volume as long as possible, and if you need to you can just reboot back and run that.

It's just a nice safe way to do major macOS upgrades vs. doing in-situ upgrades, these system collect so much crap on them, especially all the per-user preferences and application support crap stuff in ~/Library and old drivers etc. A full clean install of macOS and apps and drivers, and moving across old sessions/documents etc. if they are on the old boot volume.