I do the same thing with a kindle. I've never had it connected to the internet or used any amazon services with it. All my books were just moved over via usb.

The weird thing is how huge Calibre is considering, I'd wager, 90% of people (myself included) just use it to convert books and never touch 1/100th of the tools and functionality in it, not touching on the fact that it's not a shining example of intuitive software. But once you have it setup, using it as a middleman is pretty straightforward and easy.

Is there a simpler conversion tool that does as good of a job? I've literally not looked in a decade plus.