OK, so I tried to install Catalina to an external thumb drive. Downloaded the installer, made a bootable USB with the installer on it. Let's call that Little Thumb. Then detached all of my external devices to make it simple and booted from the USB installer. Inserted a 256GB USB3 thumb drive (Let's call that Big Thumb) to a USB port, formatted it HFS+ just to make sure it had GUID as well, then pointed the installer to Big Thumb and let it install. About 45 minutes later it wanted to know if I wanted to migrate and I said yes, then pointed to the internal drive of the MBP for the source. Let it run overnight. In the morning, had a long series of messages about files that couldn't be moved and one message that said the user name (of my account) couldn't be created. Cue the ominous music. Answered some questions about iCloud, Siri, etc, then the system finished booting from the Big Thumb. It had my user name to log in, so I'm feeling slightly better. Got it booted, but when I went to Documents, nada. Tried Applications, none of my third party software was there, no music, no pictures, no files, nothing.
So, at that point I quit. I'm not going to risk any upgrade to this can of worms until I have some concrete evidence that it will actually migrate data. Given it takes a full day to do the migration, I'll give it a go about once a week, maybe.
This is the first time Migration Assistant has failed for me. I was surprised, as none of the files listed in the warnings were things I would have though critical--files in trashcan, some plists, a few configuration files. Nothing about any applications or any data files from my account.
Has anybody done a clean install and then used Migration to get their data to the drive on an external drive? Is there something hidden that I need to do/check/uncheck/fix/sprinkle fairy dust on?