If you use the Internet--and I'm guessing you do, given you post here :-)--and have a smartphone, and have an email provider, to varying degrees you are using the 'cloud' already. You have data, both sensitive and non-sensitive, passing through someone's servers, or sitting in their database. Even just your ISP has a lot of data about your online life. Pretending that you are somehow isolated from the cloud, just because you don't sync your books or reading data on Amazon's servers, is a bit of wishful thinking IMHO. Just like the data on your smartphone and your computers, you can be more or less secure with your data in the cloud, depending on what providers you use, what apps you use, and what practices you follow. For instance, you can use one of the big cloud file store providers like Onedrive or Dropbox, and be completely secure and private with your data IF you setup client side encryption so that your files are encrypted LOCALLY before they are sent over the wire. Then, it doesn't matter if you have a snoopy provider that scans your files for whatever purpose--they cannot read them even if they wanted to. And if you are super paranoid and want to be REALLY secure, you should be encrypting all your local data that you care about too, if those computers are connected to the Internet. Bottom line: IME cloud services need not be any more or less inherently secure, or invasive of privacy, than your own devices. It's what YOU do that makes it secure, or not.

That said, I don't blame you for wanting to keep your reading data private from Amazon, it's a perfectly reasonable choice. For myself, and apparently a few others who enjoy using Amazon's cloud services for reading data, it's quite convenient and a nice toolset for reading. I don't mind Amazon knowing what I'm reading at this point (if I ever need to read something that's THAT private, I'll just read it in paper or keep it offline and out of Amazon's servers, maybe create a separate "offline books" library in Calibre if it becomes a big need--so far, I don't have a single book like that). In fact with my regular book collection, I'm considering opening up my reading notes onto Goodreads intentionally to be more 'social' with my reading.