Calibre warns about dual mobi. It recomends Mobi Old for older Kindles and AZW for newer ones. That works perfectly.
However the Cloud WILL fail. Two big outages on MS this last 10 days. I have an IT background so I wrote a fantasy explaining why stupidity rather than Cyber attacks makes WhisperSync and Cloud a bad idea. Also there are the privacy and copyright issues with Amazon, Google, MS, Dropbox etc as well as security.
Why the "Cloud" and "Whispersync" is a disaster sooner or later.
Inevitable.
Kindle Only:
https://www.amazon.com/dp/B06Y26M8Z6 I recommend download via PC, you get a different format via whispersync on some Kindles.
Kindle and all other readers:
https://www.smashwords.com/books/view/716453
No DRM even on Amazon version. You are on your honour not to upload or copy (excepting backups), which also applies to free titles NOT in the Public Domain.
I have phone App, Tablet App and five eReaders of my own I update over USB. Also load eBooks on four other Kindles and one ePub reader via USB from Calibre (100s of books from Gutenberg and now 22 of my own books & updated versions).
Backup any content you buy.