The output you provide is quite helpful. Thanks. The first run failed with an error due to the device disconnecting. However, the second run, starting at line 59, was successful! The other runs were failing, though.
However, I think you're flashing the wrong file. Luckily it's not big enough to harm the flash sections following the kernel, but now I can definitely tell you that your kernel is broken (i.e. no kernel at all anymore). You need to extract update packages with kindletool (
https://github.com/yifanlu/KindleTool), _within_ them is a raw kernel image (file named "uImage", without extension). As there was no easy way of flashing a raw kernel image before, both yifanlu and seaniko provided theirs as update packages.
So you did go the absolutely correct steps here, just using a wrong file to flash. I've been there, I flashed a lot of crap even to address 0x00000000, luckily, it is nicely recoverable.
I'm happy to see that even for a device in a tight reset loop there seem to be "windows of opportunity" to at least flash a full kernel image size's worth of data. That's actually nice.