From this now-solved issue we can garner a useful piece of information on locating corrupt files.
Even mp3 files are generally quite large. There shouldn’t be any “tiny” (ie under 10kB or so) mp3 files.
In my case, the problem with ingestion of my library was that some corrupt mp3 files were stalling the process. Probably they failed to copy from their original source and I was left with a tiny garbage file with the right name and extension.
If you are getting the ingestion process sticking, do a search for mp3 files of under 10kB in size. You can safely trash them as virtually no audio file will be that tiny.
With the help of Support I found the first two. Having determined what to look for, I found three more. Removing them allowed my library to be ingested successfully.