No, Amazon took away the “Download and transfer via USB” option when downloading your books. Making it more difficult to strip DRM, but also to conveniently add it to multiple devices.

Regarding sideloading EPUBs with Calibre, it’s not really doing that. Under normal circumstances Kindle can’t read EPUB format at all, so Calibre will convert it to a native format, either MOBI or AZW3 generally. But with a jailbroken Kindle and KOReader, you can actually read EPUBs natively. I’ve actually converted my entire collection to EPUB and deleted the AZW3 and KFX files.