Currently I'm evaluating Battery Pal[2], because the TP4057 Module was not stable enough to power USB-C to 3.5mm Adapters reliably. So far it seems to work as expected.
I wrote about my experience in cross compiling zig on an old kindle some time ago.
Have opted for other devices like the xteink (or a boox in the future) due to what seemed like a relatively small ecosystem around “aftermarket” kindle modifications.
The kindle would be a great option if it could be reliably jailbroken and loaded with custom software
source: https://kindlemodding.org/jailbreaking/post-jailbreak/disabl...