Solved. Super rusty on solving these types of problems so I left out what was probably the biggest clue. Cloning the one drive to the other was one of the first things I tried, but the software I tried (from memory, Acronis, Macrium and Easus To-Do) failed because the bytes/sector were incompatible (512 on old vs 4096 on the new) - and I assumed that problem was insurmountable. Then I go spend 15 hours trying to tackle the problem in different ways. No luck from all of the above attempts. Then I realize it the cloning isn't insurmountable, so I find a free software called HDClone that advertised itself as being able to clone in spite of the byte/sector issue. After about 3 hours the cloning was complete. Tested it a few different ways - once without the original SSD, then with the original SSD but a new file on the desktop - but this nightmare is finally over. I'd still be curious for a deeper "why" simply installing Windows itself wasn't working, but I'm content with this, even if I have a lot of cleaning to do.