Not OP but was in a similar situation sans jailbreak. I switched to a Boyue Likebook Mars which has android, micro SD card, headphone jack for audiobooks.

I installed koreader and synching so when I drag n drop ebooks or audiobooks into a certain folder, I know it will just magically appear there on the sd card of the e-reader in a few minutes.

I can still run and use the Kindle app for my existing library, though these days I buy only drm free ebooks as well.

So nice to actually own ones ebooks!