2 possible issues:
1. Did you follow step 8) of the guide? Erasing your /var/local partition? A full /var/local has weird consequences.
2. Do you have a directory "fonts" in the user mountable folder (/mnt/us/fonts)? If yes either rename it or install this version of libfreetype:
https://www.mobileread.com/forums/sho...91#post1998991
Your brick sounds like the one I had, USB-mounting worked but it was flaky. Since 5.1.0 the KT scans /mnt/us/fonts for fonts and if you have a font that makes the buggy amazon-supplied libfreetype crash -> semi-brick.