I am in the same boat. I have a Kindle 8, with firmware version 5.9.4.
I understand there is no software jailbreak for this version of firmware (see the
wiki). I think the only option, at this stage, is to use the serial jailbreak method. This involves opening up the Kindle, soldering and running commands. I'm afraid that I might stuff up my Kindle, knowing my skills with the soldering iron.
This
thread refers to jailbreaking the Kindle Paperwhite 3 with 5.9.4 using the serial jailbreak method, but I think the same applies to the Kindle 8. No guarantees it works for the Kindle 8 though. I haven't seen any reports that it does.