@
manawydan - copy the the database (metadata.db) somewhere, then browse the author & book tables (in the copy) with something like SQLite Browser, if it barfs and produces some diagnostics then they
may be useful in helping you recover, so squirrel them away somewhere.
If you have to rebuild from scratch then do it in batches, as I gather you did last time - but this time at the end of each batch backup your library. Seems like you have plenty of local disk space so I suggest you avoid cloud based solutions for now. If you use something like Freefilesynch (FFS) it only copies stuff that's changed, FFS has a good reputation for both function and performance.
Is the disk drive on which you have the library formatted for NTFS - I'm not sure why, but the thought of a 112GB library on a FAT formatted disk is a bit...
BR