You could however write a very short shell script that would do that for you. I read loads and loads of PDFs on my Kindle, and I’ve got a very simple script kin

You could however write a very short shell script that would do that for you. I read loads and loads of PDFs on my Kindle, and I’ve got a very simple script kindlecopy.sh that copies them to an appropriate (newly created if necessary) directory on the Kindle, based on the PDF metadata. That PDF metadata, too, can be edited with much lighter-weight tools than Calibre. Entrusting Calibre with doing all this work would mean much slower performance. I still use Calibre for copying the occasional EPUB/MOBI to the Kindle, but I could replace that, too, with a shell script. Calibre’s ebook-convert is a standalone program, you could script that without ever opening the Calibre library-management application.