Hello everyone!
For those interested, I have created a shell extension for Windows 10/11 which allows to import books directly from Explorer context menu.
More importantly it allows to select into which library to import, all from the same menu entry.
You can find it here: Calibre Import shell extension
I created this as I was frustrated in having to switch library in Calibre every time I needed to import groups of books into different libraries.
At first it was just a batch script but then the project sort of started flying...
I think it works rather well, it's got its own easy installer, and it's already localized in a few languages.
It is only missing the most important thing: users, and their feedback!