If you fill out the "Sharing books by email" in Calibre's settings, you can just right-click a book -> "Connect/share", "Email to [email protected]". It'll do the conversion for you (if necessary).
I'd advise a separate email address for this functionality.
It then pops up everywhere I have a Kindle app, mostly my iPad and Galaxy S9.
Huh. Now that I'm thinking about it, it seems like this would be even easier to do from the command line...