Progress, of sorts...
I managed to get my Calibre library loaded back into the 'Books' directory without the Mimas going completely catatonic.
It seems the real killer may be not so much the volume of files being indexed as the number of
large PDFs, which seem to choke the process that generates the preview thumbnails for the Recently read/Recently Added sections. I was able to load hundreds of .epubs at a time, then only tens of larger PDFs .. but still, all sorted now with 3100 files imported. The Boyue home page is reasonably responsive, and (so far) hasn't been hanging switching between apps.
The only problem is that the 'Books' tab still takes a
long time to load, at least two minutes or more while hanging the Boyue launcher, so I'm avoiding that. Since I'm using Calibre Companion, I'm hoping I can use that instead. I only really wanted the Recently read and Recently Added lists, and if you explicitly get Calibre Companion to override your default reader and open with the 'Boyue OS' app, the books show up in the 'Recently read' section.