On February 26th, Kindle customers will lose the ability to download eBook purchases directly to their PC. If you want to switch to a rival eReader brand in the future, I suggest that you use the soon-to-be discontinued “Download and Transfer via USB” feature to archive your Kindle library.
I use calibre for my kindle, but kavita for web reading on any of my devices.
The calibre web server kept claiming its downloads to my device were corrupted and would just never open books. Kavita just sends the books page as a web page which gets rid of that particular issue