Just bought a fresh (unopened) Fire TV from November 2014 - how to approach?

Hey guys,

I never thought it could happen but I got my hands on an unboxed Fire TV from November 2014. (as the seller said it was preordered.. so it should have a very old firmware on it).

I'd like to root, bootloader unlock and put custom recovery on it, anything I have to avoid except not allowing the thing to OTA update?

And how can I get root the fastest and the most risk-free way? Block ota through router and then put towelroot on it asap I guess?

Manually flashing 6.0, will this approach still work?

To start: 5.1.1 (LMY48M), rooted, with TWRP for recovery

For every 5.x update so far, this has been my procedure using fastboot:

1. Flash boot.img


2. Flash radio.img


3. Flash system.img

After successfully updated, I'd use WugFresh's Nexus toolkit to root and flash TWRP.

I'm a fan of not needing to flash userdata.img, as I don't think that should be necessary given that the OTA wouldn't wipe installed apps and such.

I've read other threads about using fastboot to upgrade and am concerned about the boot warnings, which seem to require a custom kernel to circumvent. My question is: will the above approach still work? Do I need to flash any other files? If I need to flash a custom kernel, where do I get it, how do I flash it (is it just a custom boot.img?), and at which point in the flash sequence should I apply it?

Secondly, has anyone tried gaining root through the Nexus toolkit once upgraded to 6.0?

I'm looking for recommendations so I don't spend hours trying to unbrick my device. Thanks!