Keeping wifi disabled does wonders for getting away from Amazon’s ecosystem. I side-load all of my books (obtained from non-Amazon sources) via USB, and without any internet connections to Amazon, they have no access to my Kindle and no control over it.

Not true at all. I’m on my second Kindle, and I’ve never purchased a book through Amazon or used any of their services to load books to it. I’ve always just plugged it into my computer via USB and either manually moved books to it or used Calibre as other’s have mentioned.