Import finally finished. The issue was that in Windows, behind the TurboTax Business app window, another window had launched asking for permission to access the file. It wasn't until I minimized the window to look at something else that it became visible. Once that was done, the initial import of just company info was very quick.
So lesson learned here: If your import is hanging, start minimizing windows to see if TT launched some hidden interactive panel that you need to respond to.