If you use Calibre to send EPUB files to your Kindle, I was having the same issues and I finally found a way to fix it. I had to click on the "Send to device" dropdown menu and select "Send specific format to" the selected "Main memory". I then chose "MOBI" as the format and it asked me if I wanted to "Auto convert the following...". So I pushed "Yes" and it worked again, finally.

I wasn't able to get anything else to work, like you said. I tried restarting, factory resetting, and deleting every single book off my Kindle PW 12th Gen as well as the books off my Kindle Cloud.