Last I checked, using Kindle For PC on a windows (virtual) machine still works. I did it last weekend and the instructions are still on reddit.
In the process of re-verifying and ever so slowly using kvm on my desktop and will likely duplicate the instructions as part of that. But if you just go check the calibre board at reddit, it is super easy to find.
But yeah. Strongly suggest migrating to a different ecosystem. There is no guarantee that Rakuten aren't going to shit it up but they are way better than Amazon right now (super low bar...). But since some of my favorite authors only self publish on kindle for pricing reasons...