Can't root newer kindle 10

I just got a kindle fire hd 10 and I can't seem to downgrade it from 5.1.4. It seems this one has shipped with e3 recovery which performs a signature check and causes any .bin file less than 5.1.4 to fail the signature check when sideloading. I tried 5.1.2 as others mentioned success, but no dice on mine.

Anyone have a workaround? Is there any way to fake the sig check?

You guys that had success, did your device have e3 recovery on it?

Any news for a newer version of the V10 yet?

Has anyone heard any rumors regarding if they'll be a second version of the V10 released?

Sent from my LG-H901 using Tapatalk

Unable to flash custom ROMs newer than 4.4.4

So I have an old SPH L710 that received its last OTA update before June 2014. Its 4.4.2. I've since moved on to another phone and this S3 is strictly for WiFi at home. No service provider. I'm a tech nerd building and repairing computers for the last 25 years so naturally learning how to gain root access and tweak got my attention. I've been a XDA lurker since 2012 BTW and have learned a lot just by reading. Today I need help that I can't seem to find by reading.

About a month ago I thought it would be fun to root my S3. Towelrooted, flashed Super SU, removed Knox and updated binary. Then I flashed Busy Box. All went very smooth and off I went to play and learn by reading these awesome forums.

After a couple a couple if weeks I decided to try some custom ROMs. I installed Odin on my laptop and TWRP on my SD card. Flashing TWRP via Odin went fine. Then I tried several ROMs that are specifically for my device. I wanted to try some of the Lollipop or Marshmallow ROMs. I followed the instructions exactly as I should, but I couldn't get any of them to work. Every ROM I tried put me in a boot loop. I was seeing unusual text in TWRP like, couldn't mount data and md5 not found while they were in the process of flashing.

So I decided to look for some older ROMs and found SlimKat 4.4.4
Followed the instructions and flashed ROM with Gapps usingTWRP and it worked! I don't understand why I can't flash newer ROMs on this phone. Is there something I've overlooked or is there additional files I need to flash first?

Sorry for the long post. Its late and I really want to try a newer ROM. SlimKat is awesome. My battery lasts MUCH longer and system is zippy. The only thing I don't like is Superuser is included with the ROM. Super SU is what I prefer, but unfortunately I'm stuck with it I until I find a solution to successfully flash a more up to date ROM.

Q's about repacing motherboard. Newer model? Different carrier?

So i recently hard bricked my h810 g4. (backstory post #2)

I was looking into replacing just the motherboard. I came across some cheap ones for t-moblile. Which would be nice because thats the service i use plus they are more rootable. Even it i get a blocked one, If i can get it to boot up i can sell it for parts with proof that it actually works.

Is it possible to swap the motherboard with one from a different carrier?

Also my phone was a beta phone. Serial starts with 503, so im wondering if i can even replace it with another h810.

Any advice would help.

Thanks