If you want to break free of an ecosystem / format you can go to a brand like Boox or Pocketbook. I have a pocketbook and like it, but some people think they are klunky/laggy (I wanted a smaller, water resistant reader).
I moved away from Amazon because I didn’t like the idea that they could delete or change a book I had paid for and I hated the ads for books/store on my e-reader.
If you go this route you will likely need to get Calibre free software to load books on your reader, so it does depend on how technical you are prepared to be. Not a steep learning curve but there are some recommended plugins, especially if you want to move books that you have bought on the Kindle Ecosystem. You can also go deeper and do things like have the software auto-estimate page # and reading difficulty, as well as implement tagging/organization systems, change/optimize covers, etc.